EVOLUTION 291 



represent living forms that have deviated least from the first 

 living things to be evolved from nonliving matter. 



The evidence supporting the doctrine of evolution consists of 

 a body of facts derived from morphology, embryology, physi- 

 ology, ecology, and paleontology, for which evolution seems to 

 be the most plausible explanation. This evidence is largely 

 circumstantial but so convincing as to meet with acceptance 

 among biologists and others who give the matter serious thought. 

 At the same time the truth of the evidence is not experimentally 

 demonstrable because it has not yet been possible to change one 

 species into another, i.e., to bring about experimentally the 

 evolution of a lower form into a higher one. Failure to demon- 

 strate species change experimentally detracts very little from 

 the value of the principle of evolution as a practical means of 

 interpreting biological phenomena, inasmuch as an evolutionary 

 interpretation gives meaning to otherwise inexplicable facts of 

 biology and is the basis of the scientific organization of the 

 subject. 



Variation. — Animals are classified into different species largely 

 on a basis of morphological characters. A species is composed of 

 organisms that resemble each other more closely than they do 

 other organisms. It is a group of similar individuals. Similar 

 species form a genus, similar genera a family, and so on, up to 

 the -phylum, which is the largest subdivision of either the plant 

 or animal kingdom. The species is the basic unit in classifica- 

 tion, but actually it is a variable unit because its members, 

 though closely resembling each other, are rarely identical. This 

 variability is the basis of evolution and because of it there is 

 often difficulty in deciding whether extreme variants belong to 

 the same or to different species. This is a familiar experience 

 of any one who has collected sea shells and then arranged the 

 shells of each species into a graded series according to color or 

 structural differences. Often the extremes of such a series are so 

 different that had none of the intermediate forms been found, 

 there might have been some question as to whether they really 

 belonged to the same species or not. Such lability in species 

 characters suggest that a species is not a permanently fixed unit 

 and, as a corollary, that evolution or change has taken place in 

 the past and may be taking place in the present as a result of the 

 instability of the species unit. Difficulties arising in connection 



