602 THE BEGINNINGS OF LIFE. 



(as they always must) amidst a frequently varying set 

 of external agencies, also appear as the principal causes 

 of that tendency to undergo progressive development 

 which has now been so fully demonstrated amongst 

 lower organisms, but in whose existence Mr. Darwin 

 did not believe ^ 



This absence of a belief in progressive development, 

 as well as some untenable doctrines concerning the 

 rate of change in lower organisms, seem almost wholly 

 traceable to the manner in which Mr. Darwin has 

 approached the general doctrine of Evolution. In 

 working out his theory, he has principally directed his 

 attention to higher organisms, and though his labours 

 have been crowned with so much success — though with 

 the intuition of genius he . has unravelled one of the 

 most complex problems which the evolutionist has to 

 face — still it is in this sense principally that he can 

 be considered an evolutionist. He is an evolutionist 



of his book (' Genesis of Species ') Mr. Mivart calls attention to the 

 various resemblances existing in nature between the structures of organs 

 or parts whose presence only in some of the more specialized forms of 

 totally distinct types, leads us to believe that they must have been 

 formed in a wholly independent manner. The recurrence of similar 

 structures in this way seems to show an inherent fitness in them for the 

 performance of certain functions under certain conditions, and at the 

 same time to demonstrate the ever-active influence of two great deter- 

 mining causes of form and structure. 



^ In the main the tendency is one towards ' progressive ' develop- 

 ment, though the general tendency to increase in complexity is 

 frequently interrupted in the most irregular manner by retrograde 

 changes, answering to processes which I have classed under the 

 head of Analytic Heterogenesis. 



