254 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



hypothesis is advocated in order to bring light or elucidation 

 into any field of systematic facts. And this additional 

 hypothesis indeed must be made from the very beginning, 

 quite irrespective of the more detailed problems of the law 

 of transformism, in order than any sort of so-called ex- 

 planation by means of the theory of descent may be possible 

 at all. Whenever the theory that, in spite of their 

 diversities, the organisms are related by blood, is to be really 

 useful for explanation, it must necessarily be assumed in 

 every case that the steps of change, which have led the 

 specific form A to become the specific form B, have been 

 such as only to change in part that original form A. 

 That is to say : the similarities between A and B must 

 never have become overshadowed by their diversities. 



Only on this assumption, which indeed is a newly 

 formed additional subsidiary hypothesis, joined to the 

 original hypothesis of descent in general a hypothesis 

 regarding the very nature of transformism only on this 

 almost hidden assumption is it possible to speak of any 

 sort of " explanation ' which might be offered by the 

 theory of transformism to the facts of geography, geology, 

 and biological systematics. Later on we shall study more 

 deeply the logical nature of this " explanation " ; at present 

 it must be enough to understand this term in its quasi- 

 popular meaning. 



What is explained by the hypothesis of descent in- 

 cluding the additional hypothesis, that there always is a 

 prevalence of the similarities during transformism is the 

 fact that in palaeontology, in the groups of island and 

 continent faunae and florae taken as a whole, as well as in 

 the single categories of the system, the similarities exceed 



