ORIGIN OF THE SPECIFIC TYPE 341 



mingling two species of clover, may go on reproducing with its own 

 kind for a considerable time. A number of Phanerogams yield fertile 

 hybrids, and in Orchids even species of different genera have been 

 crossed and have yielded offspring which was in some cases success- 

 fully crossed with a third genus. 



If these facts prove anything it is that the factors which 

 determine the mutual sterility of species are quite distinct from 

 their morphological differences, in other words, from the diagnostic 

 characters of the specific type. For a long time the verdict on this 

 matter was too entirely based on observations made on animals, among 

 which mutual sterility arises relatively easily, even where it was 

 not intended (sit venia verbo ! ). Even the pairing, but still more the 

 period of maturity, the relations of maturity in ovum and sperm, and 

 even the most minute details in the structure of the sperm-cell, the 

 egg-shell, envelope, &c., have to be taken into account, and these may 

 bring about mutual or, as Born has shown, one-sided sterility. We 

 know, through the researches of Strasburger, that a great many 

 Phanerogams, when pollinated artificially from widely separated 

 species of different genera and families, will at least allow the pollen- 

 tube to penetrate down to the ovule, and that in many cases 

 amphimixis actually results. It follows that we must not lay too 

 great stress upon the mutual sterility which occurs almost without 

 exception among the higher animals, but must turn to the plants with 

 greater confi.dence. 



Among plants there is very widely distributed mutual fertility 

 between species. I doubt, however, whether the observations on this 

 point are sufficient to warrant any certain conclusion in regard to the 

 importance of the phenomena in the formation of species. At least it 

 is not easy to see why the mutual sterility of many species of plants 

 should not have been necessary or useful in separating species, and 

 why it was not therefore evolved. We may point to the fact that 

 animals can move from place to place as the chief reason, and this 

 factor does undoubtedly play a part, but the widespread crossing 

 of plants by insects makes up to some extent, as far as sexual 

 intermingling is concerned, for their inability to move from place to 

 place. I do not know whether the species of orchid which are fertile 

 with one another belong to different countries, so that we may 

 assume that they originated in isolation, or whether fertile orchids 

 from the same area are fertilized by different insects and are thus 

 sexually isolated. This and many other things must be taken into 

 consideration. Probably these relations have not yet been adequately 

 investigated ; probably what is known by some experts has not yet 



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