52 DARWINISM TO-DAY. 



be explained by the selection theory only when there is 

 postulated a definite degree of complexity of the fortu- 

 itously appearing slight variations, that is, when this 

 theory is in condition to assume that which would be a 

 denial in terms that variations are wholly fortuitous. 

 Wolff goes on to give certain examples of such complex 

 relations which involve a dependence of the use upon an 

 instinct, as the performance by the queen honey-bee of her 

 particular functions in the hive, etc. 



"Out of this discussion," says Wolff, "finally we must 

 postulate that structures which are to be explained by the 

 selection theory must possess at the least two certain charac- 

 teristics. Such a structure, namely, must occur but once 

 in an organism [that is, must not be a serially or bilaterally 

 repeated organ, nor indeed appear in any condition of plural 

 number] ; further, it must not stand in any necessary relation 

 to any other part of the same organism, that is, in a relation 

 which one can interpret as a relation not existing from 

 the beginning, therefore one which must be looked on as an 

 acquired relation. But if we survey the whole animal king- 

 dom it will be very difficult for us to find any structures 

 which satisfy both these requirements. It might be possible 

 to find some which perhaps seem to satisfy the second re- 

 quirement, but with regard to the first requirement I may 

 declare," says Wolff, "that there is scarcely a single structure 

 which fulfils it. Symmetry alone, which rules almost all 

 organisms, makes organs which appear in the singular num- 

 ber rarities, and even such as the pancreas, etc., are com- 

 posed of many finer structures, which are homodynamous 

 among themselves. When we find two similar organs in 

 different animal groups we seek for a causal explanation of 

 this similarity and find it in common ancestry. It is absurd 

 to seek a causal explanation for the origin of homologous 

 structures and yet postulate a purely chance or fortuitous 

 explanation for the origin of homodynamous structures." 



