i86 COMPARATIVE ANATOMY 



the group from which both animals and plants have sprung, 

 but this we can hardly do. Were such a supposition true we 

 should expect the group to be still giving rise to new forms of 

 animals and plants; for if it was capable of evolving higher 

 forms in the past, why not in the present and the future ? We 

 have no evidence that the Flagellates are giving, or have 

 recently given birth to new forms of higher organisation, and 

 we must regard the living representatives of the group as 

 possibly representing the early forms of life from which animals 

 and plants sprung, but themselves so far specialised and 

 adapted ' to particular conditions of life as to be now incapable 

 of evolution outside the limits of their own special organisa- 

 tion. The Volvocinse certainly suggest to us the successive 

 steps through which the multi-cellular condition was evolved 

 from the uni-cellular, and the importance of such a series 

 cannot be over-estimated. But again, if we would be scientific 

 we must be cautious, and we must not say that these are the 

 steps through which Protozoa developed into Metazoa or 

 Protophyta into multi-cellular plants. There is, in fact, no 

 evidence that Volvox leads us any farther in the scale of 

 organisation, and, indeed, there is some reason to suppose that 

 in the Volvocinse we have a line of evolution which has reached 

 its acme in Volvox itself. The Flagellata suggest much, but 

 they prove little, except that there are no distinct boundaries 

 in living nature. A group which merges on the one hand 

 into the Rhizopoda, on the other hand passes by insensible 

 gradations into the multi-cellular condition, a group which 

 belongs neither to the animal nor to the vegetable kingdom, 

 but equally to both, brings to our minds the conviction of the 

 unity of nature. And this idea, of the unity of organic life, is 

 one of the most important in enabling us to apprehend the 

 full meaning of the doctrine of Evolution. 



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