_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