372 THE CHANGING GENERATIONS 



with distinguishing among organisms and hence tends to emphasize their 

 differences. 



The description and naming of organisms began long before the doctrine 

 of evolution was established. Since then taxonomy has become as much 

 concerned with the discovery and expression of evolutionary relationships 

 as with the description and naming of species. The system of classification 

 rests upon the assumption that degrees of homologous resemblance 

 correspond with degrees of relationship. A hierarchy of more inclusive 

 and less inclusive categories is erected to express these degrees of rela- 

 tionship. The fact that living things fit into these categories and that we 

 do not find organisms showing a jumble of inconsistent characteristics 

 is understandable in the light of our preceding discussion of the meaning 

 of homology. 



Species and genus. We often speak of "kinds" of animals, without 

 any very precise meaning. Sometimes we use "kind" to mean a group of 

 similar individuals, such as cows, or sheep, which breed together and 

 form a population in a restricted and definite sense. At other times we use 

 "kind" more loosely, to include broader groups such as snakes or fishes, 

 which are obviously of more than one sort. In biology it is necessary to 

 distinguish between the different kinds of "kinds" — to set up definite 

 systematic categories of greater and lesser inclusiveness. 



The basic systematic unit is the species. This term (spelled alike in 

 singular and plural) is applied to populations of closely similar individuals, 

 such as men, or English sparrows, or bullfrogs, which, in general, are alike 

 in most morphological, physiological, and embryological features, reproduce 

 among themselves, and are of common descent. 



Actually it is very hard to give a good definition of species that is 

 universally applicable. In many plants and some animals there is an alter- 

 nation of generations (sporophyte-gametophyte, sexual-asexual, or 

 bisexual-parthenogenetic), with marked differences between the genera- 

 tions. Widespread interbreeding populations of animals and plants often 

 show local or regional differences, and instances are known in which the 

 two ends of a chain of interbreeding populations are so different that the 

 end populations cannot breed with one another. In such species as the 

 dog the extremes of variation that have been produced by selective breed- 

 ing are far more unlike morphologically than distinct species usually 

 are. Even if species are hard to define, in a given group it is usually possi- 

 ble, with some experience, to tell definitely what are and what are not 

 species. The following examples will illustrate the species concept. 



Among the squirrels of the eastern United States there are two kinds 

 known respectively as the gray squirrel and the fox squirrel. Each of 

 these is a species; but both are squirrels, and hence "squirrel" is a larger 

 category that includes both species. Another somewhat similar animal, the 



