No. 579] PROGRESSIVE EVOLUTIOX 



151 



ing of organic evolution cannot all be correct in all their 

 details, but it may be that each contains its own elements 

 of truth, and that if these elements can but be recognized 

 and sorted out, they may perhaps be recombined in such 

 a form as to afford at any rate a plausible working 

 hypothesis. We must bear in mind from the outset that 

 in dealing with such a complex problem many factors 

 have to be taken into account, and that widely different 

 views on the question may be merely one-sided and not 

 necessarily mutually exclusive. 



I take it there are three principal facts, or groups of 

 facts, that have to be accounted for by any theory 

 of organic evolution: 



1. The fact that, on the whole, evolution has taken 

 place in a progressive manner along definite and di- 

 vergent lines. 



2. The fact that individual animals and plants are 

 more or less precisely adapted in their organization and 

 in their behavior to the conditions under which they have 



3. The fact that evolution has resulted in the existence 

 on the earth to-day of a vast number of more or less 

 well-defined groups of animals and plants which we call 

 species. 



The first of these facts appears to me to be the most 

 fundamental, and at the same time the one to which 

 least attention is usually paid. The great question, after 

 all, is, Why do organisms progress at all instead of re- 

 maining stationary from generation to generation? To 

 answer this question it is not necessary to go back to 

 the beginning and consider the case of the first terres- 

 trial organisms, whatever they may have been, nor are 

 we obliged to take as illustrations the lowest organisms 

 known to us as existing at the present day. We may 

 consider the problem at any stage of evolution, for at 

 each stage progress is, or may be, still taking place. 

 We may even begin by considering what is usually re- 

 garded 'as the highest stage of all, man himself; and 

 indeed this seems the most natural thing to do, for we 



