314 INTRODUCTION TO EVOLUTION 



artificial system. But the similarities forming the basis for the cataloguing 

 are now regarded as indicative of genetic relationship — related animals be- 

 ing classified together, unrelated ones separated from each other. Hence the 

 artificial system has become a natural system // we agree that fundamental 

 similarities of structure derive from common ancestry, the evolutionary 

 explanation. 



WHAT IS A SPECIES? 



Students of evolution lay particular emphasis upon the 

 small unit of classification mentioned above, the species. This is reflected 

 in the fact that Darwin named his great treatise on evolution The Origin 

 of Species. It is felt that if the origin of separate species can be accounted 

 for, the origin of genera, families, orders, and so on, can be explained by 

 the further application of the principles discovered or by extensions of 

 those principles. To a considerable extent two new species arising from one 

 original parent species would constitute the first step in evolutionary 

 change. Two men walking down a road come to a fork in that road; one 

 man follows the road to the right, the other man the road to the left. For a 

 little while the two men are near together even though they may eventually 

 have diverged so that they are thousands of miles apart. Similarly, two 

 groups of animals starting on divergent courses of evolution would at the 

 outset be very similar, similar enough to be regarded as two species in the 

 same genus, even though their remote descendants may be "poles apart" 

 in structure. Accordingly, if we can account for the first step in developing 

 diversity of structure, the step which produces enough diversity to separate 

 groups into distinct, though related, species, we have gone far toward an 

 understanding of the causes of evolution. The species, then, occupies a key 

 position in thinking on evolution. It is important that we inquire into the 

 nature of the species with a view to determining what is involved in the 

 origin of species. 



What is a species? We asked that question earlier, in our discussion of 

 human evolution (pp. 248-250). There we noted two attributes of species: 

 (1) some degree of structural difl'erence and (2) reproductive isolation. 

 From differences of opinion concerning the relative importance of these 

 two attributes arises most of the controversy over definition of the word. To 

 focus attention upon the contrasting points of view we shall quote two pro- 

 posed definitions. The first is that of Tate Regan, quoted with approval by 

 various subsequent authors (e.g.. Gates, 1948) : "A species is a community, 

 or a number of related communities, whose distinctive morphological char- 



