ii4 AN INTRODUCTION TO ZOOLOGY 



Classification. 



The frog, whose anatomy, histology, physiology and, to a lesser 

 extent, development we have now studied, will also serve to introduce 

 another important branch of Biology which follows after these, 

 namely, Taxonomy or Classification. When we have learned what 

 we can of a new animal it is necessary to give it a name, and to 

 ascertain its position with regard to other animals. The first object, 

 the naming, follows from the second, and cannot be done arbitrarily, 

 for the object of a natural, as opposed to an artificial, classification 

 is to arrange the animals in groups that show their relationships 

 with one another, and then the name given is one that will indicate 

 the position of the animal in these groups. The theory of descent 

 is the great key to this problem of classification, for it teaches that 

 structural similarity or homology indicates a community of descent, 

 and therefore a degree of affinity, and we utilise this theory to 

 bring animals together in such a way as to make clear their relation- 

 ships. It is obviously necessary, in order to carry out such a scheme, 

 to have some recognisable unit to form its basis, and so we associate 

 together all animals that possess certain fairly definite and constant 

 characters of structure and form, and this assemblage we call a 

 species. This term is hard to define accurately, for it has no 

 absolute criteria or definite limits, but, as ordinarily used, implying 

 a group of beings resembling one another, closely and constantly, 

 and easily recognisable, as belonging to one type, it is a very useful 

 concept, in spite of its vagueness. There is a certain amount of 

 variation within the limits of each species, and each animal has its 

 own individual peculiarities, but they are not so great usually as to 

 obscure the type to which the individual belongs. The constancy 

 that has been referred to is only a relative matter, for although, 

 as far as human remembrance or records go, the species of animals 

 now on the globe are constant, it will be seen that, when we bear in 

 mind the phenomenon of evolution, the constancy is more apparent 

 than real, for they must all have arisen from pre-existing different 

 types. The rate of change, however, is so slow that this alteration 

 is lost sight of. The actual differences separating one species from 

 another are often small, merely differences of colour or size and 

 proportions, as, for example, those between the lion and the tiger, 

 and yet members of one species, as a rule, cannot, or do not, breed 

 with those of another. 



When a number of species resemble one another, closely possess- 

 ing certain characters in common, we associate them together in a 

 larger group that is termed a Genus. A species then is a number of 

 individuals possessing in common certain specific characters, while 

 genera are groups of one or more species with common generic 

 characters. It sometimes happens that a species is so distinct from 



