726 GENETICS AND EVOLUTION 



from 200 to 300 feet. 1 his created land connections, highways for 

 the cUspersal of many hind forms, l^etween Siberia and Alaska at Bering 

 Strait, and between England and the continent of Europe. Many mam- 

 mals, including the saber-toothed tiger, the mammoth and the giant 

 ground sloth, became extinct in the Pleistocene after primitive man had 

 apjjeared. 



The fossil record available today makes it impossible to doubt 

 that the present species arose from previously existing, different ones. 

 For many lines of evolution the individual steps are well known; other 

 lines have some gaps which remain to be filled by futaue paleontolo- 

 gists. 



Even if there were no fossil record at all, the results of the detailed 

 studies of the morphology, physiology and biochemistry of present-day 

 animals and plants, of their mode of development, of the transmission 

 of inherited characteristics, and of their distribution over the earth's 

 surface would be sufficient to prove organic evolution. 



310. The Evidence from Taxonomy 



The science of naming, describing and classifying organisms, 

 taxonomy, was discussed in Chapter 7. The science of taxonomy began 

 long before the doctrine of evolution was accepted; indeed the founders 

 of scientific taxonomy, Ray and Linnaeus, were firm believers in the 

 fixity, the unchangingness, of species. Present-day taxonomists are con- 

 cerned with the naming and describing of species primarily as a means 

 of discovering evolutionary relationships, based upon the assumption 

 that the degree of resemblance in homologous structures is a measure of 

 the degree of relationship. The fact that the characteristics of living 

 things are such that they can be fitted into a hierarchical scheme of 

 categories, each more inclusive than the previous one— species, genera, 

 families, orders, classes and phyla, can best be interpreted as proof of 

 evolutionary relationship. If the kinds of animals and plants vvere not 

 related by evolutionary descent, their characters would be present in a 

 confused, random pattern and no such hierarchy of forms could be 

 established. 



The basic unit of taxonomy is the species, a population of closely 

 similar individuals, which are alike in their morphologic, embryologic 

 and physiologic characters, which in nature breed only with each other, 

 and which have a common ancestry. It is difficult to give a definition of 

 species that is universally applicable. The definition must be modified 

 slightly to include species wliose life cycle includes two or more quite 

 different forms (many coelenterates, parasitic worms, larval and adult 

 insects and amphibians, for example). A species which is spread over a 

 wide territory may show local or regional differences which may be 

 called subspecies. Many instances are known in which a species is broken 

 up into a chain of subspecies, each of which differs slightly from its 

 neighbors but interbreeds with them. The subspecies at the two ends 

 of the chain, however, may be so different that they cannot interbreed. 



