THE AMERICAN NATURALIST [Vol.LVI 



that it is mixing of stocks, hybridization, that is the origin 

 of organic diversity? Or do we know that the physical 

 and chemical conditions of the environment produce 

 changes that are inherited and give us evolution! Or 

 finally, do several or all of these methods of action con- 

 Such are the questions, I take it, on which we hope for 

 light in the discussion this afternoon, and in the discus- 

 sion of orthogenesis before the American Society of Zo- 

 ologists, and of the species concept before the Botanical 

 Section of the American Association. 



The lines of attack on the problem of variation are as 

 manifold as are the questions to be answered. The basic 

 idea in that attack whose results I shall try to summarize 

 is this : In the reproduction from two parents familiar to 

 us in higher animals and plants, there is a mixing of dif- 

 ferent stocks, a formation of great numbers of diverse 

 groups of the hereditary materials, with consequent pro- 

 duction of a great variety of diverse offspring from a 

 given pair of parents. This is the chief cause of the dif- 

 ferences everywhere observable among individuals: dif- 

 ferences formerly classed as variations and considered 

 the material of evolutionary change. But such kaleido- 

 scopic regrouping of materials, the units of which are not 

 changing, has no obvious relation with evolutionary vari- 

 ation ; in the next generation a new grouping of the same 

 material occurs, and so on indefinitely. If there likewise 

 occur progressive evolutionary changes, these are so lost, 

 so hidden, in the multitude of kaleidoscopic recombina- 

 tions that they can not be distinguished; the literature 

 of evolution is filled with confusion due to this difficulty. 



Therefore the idea suggests itself: Why not avoid at 

 once all this, by studying evolutionary^ changes in those 

 organisms wlii re no mixing of stocks is oconrring; where 

 there is no knNM.l-.srr.pic ivumupin- ,,1* \\u- liereditary 

 materials? 'riicr-c niv ori;;iiiisiiis tli;it reproduce from 

 a single parent, with no shifting or r('C()in})iiiation of the 

 germ plasm; in these, actual changes that persist from 



