2i 4 COMPARATIVE ANATOMY 



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 

 Flagellata 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 condi- 

 tions of life as to be now incapable of evolution outside the 

 limits of their own special organisation. The Volvocinae 

 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 Volvocinae we have 

 a line of evolution which has reached its acme in Volvox itself. 

 The Flagellata suggests 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. 



