THE SPECIES 239 



and error," a law, that is, of aimless testing in every direction 

 and of a " selection of the fit " depending on external circum- 

 stances. 



As we already know, there lies ready in every germ a 

 definite number of quite definite genes, which together repre- 

 sent the genotype. They themselves have as yet no frame- 

 work, but, through the ordered advent of impulses, they are 

 enabled to produce it. 



Through the crossing of different individuals of the same 

 species, these genes are reciprocally exchanged, in accordance 

 with Mendel's rule. 



If we consider the crossing, within a species, of individuals 

 that vary very much from one another (as in the fly 

 Drosophila ampelophila, which we know from the fine work of 

 Morgan and his pupils), we see that, compared with the sample 

 animal, the species possesses a much greater number of genes, 

 many of the properties of which absolutely contradict one 

 another. As well as genes for all conceivable kinds of eye- 

 colour, we also find genes for eyelessness. Likewise there are 

 genes for certain shapes of wings, and others for wingless- 

 ness ; and so forth. 



In spite of the extraordinary wealth in genes, yet from 

 every crossing that yields a living germ there arises an indi- 

 vidual capable of functioning, which we call a Drosophila 

 ampelophila. 



The same thing appears within every species. Even 

 Paramecium forms species, the individuals of which diverge 

 from one another in every direction, and nevertheless all 

 belong to one unity, which we call species. 



Starting from this fact, the species has been defined as 

 that number of different individuals which, crossed with one 

 another, continue to produce offspring capable of living and 

 of reproducing themselves. 



If we accept this definition, then the species consists of 



