216 GENERAL EVOLUTION. 



members ; organic types miglit have been created unchangeable, 

 but presenting the mutual relations in question. But if transi- 

 tions among these members can be shown to take i^lace, then 

 indeed the phenomena mentioned received a sufficient explanation. 

 They are seen to be the necessary relations of the parts of a shift- 

 ing scene of progression and retrogression ; they express combina- 

 tions of structure, which, though often long enduring, are, never- 

 theless, not perpetual, but give way to other combinations to be 

 in their turn dissolved. Now, if there is anything well known in 

 nature, it is that there are divisions of various ranks in the vege- 

 table and animal kingdoms, whose contents present variations of 

 structure which are confessedly additions to or subtractions from 

 the characters of ancestors, which have appeared during ordinary 

 descent. The j^rotean species, genera, etc., are well known to 

 biologists, and every naturalist who admits varieties, sub-species, 

 sub-genera, etc., admits derivation so far as they are concerned. 

 The facts of variation, including ''^sporting," etc., are notorious, 

 not only among domesticated, but also in wild animals and plants. 

 The facts have led some persons to suggest that species have been 

 produced by evolution from a single specific center, but that the 

 genus and other comprehensive divisions are unchangeable. But 

 I think I have shown, in a paper entitled " The Origin of Gen- 

 era," * that the structural characters which define genera, and even 

 higher divisions, are subjects of variation to as great an extent as 

 are the less profound specific characters ; and, moreover, that the 

 evidence of derivation which they present is singularly clear and 

 conclusive. The changes of both genus and species character are 

 always of the nature of additions to or subtractions from those of 

 one generation displayed by their descendants. As such, they 

 form the closing chapters of the embryonic or growth-history of 

 the modified generation. 



In order to explain more fully the application of the above 

 statements, I introduce a few examples selected from the subjects 

 of my studies. Their number might be indefinitely extended. I 

 first cite the genera of the tailless BatracMa Anura (frogs, toads, 

 etc.), whose relations are very simple and clear, and show the 

 parallelism between adult structure and embrj^onic succession. See 

 above, 1 and 2. 



The greater number of BatracMa Anura fall into two divis- 



* Philadelphia, 1869. "Proceedings Academy Natural Sciences, 1868." 



