Problems of Genetics

CHAPTER I. Introductory

INTRODUCTORY

The purpose of these lectures is to discuss some of the familiar phenomena of biology in the light of modern discoveries. In the last decade of the nineteenth century many of us perceived that if any serious advance was to be made with the group of problems generally spoken of as the Theory of Evolution, methods of investigation must be devised and applied of a kind more direct and more penetrating than those which after the general acceptance of the Darwinian views had been deemed adequate. Such methods obviously were to be found in a critical and exhaustive study of the facts of variation and heredity, upon which all conceptions of evolution are based. To construct a true synthetic theory of Evolution it was necessary that variation and heredity instead of being merely postulated as axioms should be minutely examined as phenomena. Such a study Darwin himself had indeed tentatively begun, but work of a more thorough and comprehensive quality was required. In the conventional view which the orthodoxy of the day prescribed, the terms variation and heredity stood for processes so vague and indefinite that no analytical investigation of them could be contemplated. So soon, however, as systematic inquiry into the natural facts was begun it was at once found that the accepted ideas of variation were unfounded. Variation was seen very frequently to be a definite and specific phenomenon, affecting different forms of life in different ways, but in all its diversity showing manifold and often obvious indications of regularity. This observation was not in its essence novel. Several examples of definite variation had been well known to Darwin and others, but many, especially Darwin himself in his later years, had nevertheless been disposed to depreciate the significance of such facts. They consequently then lapsed into general disparagement. Upon more careful inquiry the abundance of such phenomena proved to be far greater than was currently supposed, and a discussion of their nature brought into prominence a consideration of greater weight, namely that the differences by which these definite or discontinuous variations are constituted again and again approximate to and are comparable with the class of differences by which species are distinguished from each other.

The interest of such observations could no longer be denied. The more they were examined the more apparent it became that by means of the facts of variation a new light was obtained on the physiological composition and capabilities of living things. Genetics thus cease to be merely a method of investigating theories of evolution or of the origin of species but provide a novel and hitherto untried instrument by which the nature of the living organism may be explored. Just as in the study of non-living matter science began by regarding the external properties of weight, opacity, colour, hardness, mode of occurrence, etc., noting only such evidences of chemical attributes and powers as chance spontaneously revealed; and much later proceeded to the discovery that these casual manifestations of chemical properties, rightly interpreted, afford a key to the intrinsic nature of the diversity of matter, so in biology, having examined those features of living things which ordinary observations can perceive, we come at last to realize that when studied for their own sake the properties of living organisms in respect of heredity and variation are indications of their inner nature and provide evidences of that nature which can be obtained from no other source.

While such ideas were gradually forming in our minds, came the rediscovery of Mendel's work. Investigations which before had only been imagined as desirable now became easy to pursue, and questions as to the genetic inter-relations and compositions of varieties can now be definitely answered. Without prejudice to what the future may disclose whether by way of limitation or extension of Mendelian method, it can be declared with confidence and certainty that we have now the means of beginning an analysis of living organisms, and distinguishing many of the units or factors which essentially determine and cause the development of their several attributes.

Briefly put, the essence of Mendelism lies in the discovery of the existence of unit characters or factors. For an account of the Mendelian method, how it is applied and what it has already accomplished, reference must be made to other works. With this part of the subject I shall assume a sufficient acquaintance. In these lectures I have rather set myself the task of considering how certain problems appear when viewed from the standpoint to which the application of these methods has led us. It is indeed somewhat premature to discuss such questions. The work of Mendelian analysis is progressing with great rapidity and anything I can say may very soon be superseded as out of date. Nevertheless a discussion of this kind may be of at least temporary service in directing inquiry to the points of special interest.

THE PROBLEM OF SPECIES AND VARIETY

Nowhere does our new knowledge of heredity and variation apply more directly than to the problem what is a species and what is a variety? I cannot assert that we are already in a position to answer this important question, but as will presently appear, our mode of attack and the answers we expect to receive are not those that were contemplated by our predecessors. If we glance at the history of the scientific conception of Species we find many signs that it was not till comparatively recent times that the definiteness of species became a strict canon of the scientific faith and that attempts were made to give precise limits to that conception. When the diversity of living things began to be accurately studied in the sixteenth and seventeenth centuries names were applied in the loosest fashion, and in giving a name to an animal or a plant the naturalists of those times had no ulterior intention. Names were bestowed on those creatures about which the writer proposed to speak. When Gesner or Aldrovandi refer to all the kinds of horses, unicorns, dogs, mermaids, etc., which they had seen or read of, giving to each a descriptive name, they do not mean to "elevate" each named kind to "specific rank"; and if anyone had asked them what they meant by a species, it is practically certain that they would have had not the slightest idea what the question might imply, or any suspicion that it raised a fundamental problem of nature.

Spontaneous generation being a matter of daily observation, then unquestioned, and supernatural events of all kinds being commonly reported by many witnesses, transmutation of species had no inherent improbability. Matthioli, for instance, did not expect to be charged with heresy when he declared Stirpium mutatio to be of ordinary occurrence. After giving instances of induced modifications he wrote, "Tantum enim in plantis naturae germanitas potest, ut non solum saepe praedictos praestet effectus, sed etiam ut alteram in alteram stirpem facile vertat, ut cassiam in cinnamomum, sisymbrium in mentham, triticum in lolium, hordeum in avenam, et ocymum in serpyllum."

I do not know who first emphasized the need for a clear understanding of the sense in which the term species is to be applied. In the second half of the seventeenth century Ray shows some degree of concern on this matter. In the introduction to the Historia Plantarum, 1686, he discusses some of the difficulties and lays down the principle that varieties which can be produced from the seed of the same plant are to be regarded as belonging to one species, being, I believe, the first to suggest this definition. That new species can come into existence he denies as inconsistent with Genesis 2, in which it is declared that God finished the work of Creation in six days. Nevertheless he does not wholly discredit the possibility of a "transmutation" of species, such that one species may as an exceptional occurrence give rise by seed to another and nearly allied species. Of such a phenomenon he gives illustrations the authenticity of which he says he is, against his will, compelled to admit. He adds that some might doubt whether in the cases quoted the two forms concerned are really distinct species, but the passage is none the less of value for it shews that the conception of species as being distinct unchangeable entities was not to Ray the dogma sacrosanct and unquestionable which it afterwards became.

In the beginning of the eighteenth century Marchant, having observed the sudden appearance of a lacinated variety of Mercurialis, makes the suggestion that species in general may have arisen by similar mutations. Indeed from various passages it is manifest that to the authors of the seventeenth and early eighteenth centuries species appeared simply as groups more or less definite, the boundaries of which it was unnecessary to determine with great exactitude. Such views were in accord with the general scientific conception of the time. The mutability of species is for example sometimes likened (see for instance Sharrock, loc. cit.) to the metamorphoses of insects, and it is to be remembered that the search for the Philosopher's Stone by which the transmutation of metals was to be effected had only recently fallen into discredit as a pursuit.

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