THE EVOLUTIONS OF LIVING BEINGS. I47 



the class to which the crossed individuals belonged. 



But this statement is not correct. The difference is 

 but one of degree. A cross between different Linneons 

 also gives rise to differently constituted types and to 

 consider these to be of the same general plan as the 

 Linneons crossed, is again only possible by abstracting 

 from the differences. 



If we cross f. i. two white Sweetpeas — even such 

 belonging to the same Linneon — we may obtain 

 colored tjrpes, some of which have exclusively colored 

 descendants although they may segregate into many 

 differently colored types. 



Now a group of such colored types has a different 

 „groundplan" from the uncolored types from whose 

 cross they arose ; we are even able to say in what these 

 groundplans differ. They differ in so far as the color- 

 less types crossed, possess only one substance 

 of a certain class, either a chromogen or an 

 oxydase, while the colored groups, resulting from 

 their union possess both substances in their „ground- 

 plan." 



Now, mutatis mutandis, the skelettal groundplan of 

 the vertebrates may have arisen by the crossing of 

 two invertebrates, each of which possessed some of 

 the substances necessary to form a skeleton, but lacked 

 some of the others, which combined by crossing these 

 two t57pes. 



And just as in the colored Sweetpeas mentioned 

 above, the groundplan „color" remains, but is different 

 in the different segregates from some of these colored 

 types, so the groundplan of the skeleton may remain in 



