CHAPTER IX.. Proofs From Variation of Organic Forms, Artificial and Natural.

PROOFS FROM VARIATION OF ORGANIC FORMS, ARTIFICIAL AND NATURAL.

Limitation of the use of experiment in morphology--Unconscious experiments in breeding, and their results--Principles involved--Inheritance, immediate and ancestral--Effect of true breeding long continued--Method of selection illustrated by diagram--Formation of a race--Process the same in nature --Show selective effect of physical environment--Of organic environment--Of migrations--Of unlimited time--Other factors of change, and their effects shown in nature and in domestication--Differences between artificial and natural species 222

First difference, reversion--The tendency to reversion described--The reason explained--Illustrated by the case of the pointer 229

Second difference, intermediate forms--Reason is, these are eliminated in nature 232

Third difference, cross-fertility--Natural species are usually cross-sterile--Degrees of cross-sterility--Two bases of species, morphological and physiological--Two kinds of isolation, sexual repugnance and cross-sterility--Latter most essential--Illustrated by plants and hermaphrodite animals-- Former only higher animals--Natural laws interfered with by domestication--Illustrated by plants and animals 232

Law of cross-breeding--Effect of close breeding--Of crossing varieties to a limit--The law investigated--Reproduction in lowest organisms--Fission--Gemmation--Internal gemmation --Sex introduced--Effect of, is funding of differences in offspring and tendency to variation--Sexual and non-sexual reproduction compared--Separation of sex elements-- Of sex-individuals--Introduction of sex-attraction-- Funding of greater differences in offspring--Crossing of varieties--Diagram illustrating effect in vigor--Effect also in plasticity--Application of these principles-- Necessity of sexual isolation to produce species--Origin of cross-sterility and thus of species by Dr. Romanes’s idea-- Why artificial varieties are cross-fertile--Geographical species sometimes cross-fertile--Application of principles-- Absence of intermediate links in natural species explained-- Under what conditions such are found--Further explanation of this point--Illustrated by a growing tree 236

Objection answered--Intermediate links ought to be found fossil --Answer (1) Imperfection of record. (2) The term species indefinite. (3) Transitions between all other taxonomic groups abundant. (4) Between species, also, both living and fossil --Of fossil, Planorbis of Steinheim--Other examples--(5) Why transition-forms are rare--Answer--Changes in every department of nature are paroxysmal--Illustrated--So the steps of evolution paroxysmal--Critical periods in evolution --Causes of rapid advance--Apparent discontinuity between species--(1) changes paroxysmal--(2) Brooks’s idea--Male sex is the progressive element--Illustrated by society-- Effect of prosperous times--Mrs. Treat’s experiments--Hard times produce excess of males, and therefore tend to diversity --Summary 248

Objection--Egyptian drawings and mummy plants, show no change --Answer (1) Time too short. (2) We are now in time of slow change. (3) All species don’t change, most become extinct. (4) Evolution is probably slower now than formerly--Reasons for so thinking--Organic evolution approaching completion-- Other supposed objections 265

Origin of beauty--Explanation of, in higher animals--In flowering plants--But in many cases we can’t explain 269

Incipient organs--Difficulty of explaining--But these are not objections to the fact of evolution, but only to the sufficiency of the present theories of evolution. Therefore, all discussion concerns special theories. The fact of evolution is certain 270