CHAPTER X 



SOME TEST CASES BETWEEN THE 

 RIVAL THEORIES 



A. NUMERICAL 



JL T is now almost unquestioned that existing plants and animals 

 have been produced by an evolution that, on the whole, has gone 

 forward, producing organisms of increasing complexity such as 

 man and the higher animals and plants. But many of the " lower " 

 things, the seaweeds, the lichens, the smaller ferns, the insects, 

 etc., have not been killed out, but have also increased very 

 greatly in number. This has always been difficult to explain upon 

 the current theory, but is perhaps more easy of explanation if we 

 consider that evolution was not altogether a matter of con- 

 tinuous improvement in adaptation, at any rate as indicated in 

 external characters, which are almost the only things to show us 

 that there has been any great evolution at all. 



We have seen that a good case can be made out for differen- 

 tiation, in so far as it implies that a family most probably began 

 (at one step) as one genus with one species, of family rank, giving 

 rise later to other genera and species carrying the family characters 

 (but often with modifications in various directions), and making 

 in this way a family whose numbers would steadily increase, 

 inasmuch as there was no necessary reason why any of them should 

 die out, as there was under natural selection, which killed out the 

 less well-adapted ancestors. The loss of this first species and 

 genus would of course exterminate the family, but as it grew in 

 size, the loss of one genus with one species would matter less and 

 less, the rank of the genus with reference to the family becoming 

 continually less, the smaller the genus in proportion to the size of 

 the family. 



The adoption of the theory of differentiation of course turns 

 the working of the mechanism of evolution the other way round, 

 and in the opinion of the writer puts events in their proper 

 sequence. It therefore seems clear that the first thing to be done 

 is to decide which of the two views is the more correct one to 

 take. Did evolution go in the direction from variety and species 

 towards higher forms (Darwinism), or in the reverse way (Dif- 

 ferentiation)? Did the family begin as a species of family rank, 

 or was it gradually formed by the destruction of intermediates? 



