XVII THE METHOD OF ORGANIC EVOLUTION 361 



in certain groups are reduced to four, three, or two. The 

 five-jointed tarsus is, however, the most prevalent ; and in 

 the enormous order of coleoptera or beetles, comprising at 

 least one hundred thousand described species, fully half 

 belong to families which have the tarsi five-jointed. Even 

 the antennae, although they vary greatly in the number of 

 joints, yet in numerous large groups comprising many 

 thousands of species, they have the number of joints 

 constant. Another indication of the tendency of serial 

 parts to become fixed in number, is the typical limitation 

 of the cervical or neck vertebrae of mammalia to seven 

 joints. This number is wonderfully constant, being the 

 same in the long necks of the giraffe and camel, and the 

 very short necks of the hippopotamus, porpoise, and mole, 

 the only exceptions in the whole class being some of the 

 sloths, which have from six to ten, often varying in the 

 same species, and the manatee, which has six. 



Now, if we consider the enormous extent of these fixed 

 numerical relations of important parts of the organism in 

 the higher vertebrates and in insects, both as regards the 

 number of living species affected — perhaps ninety-nine 

 per cent, of the whole — and as regards their range in time, 

 throughout the whole of the tertiary and secondary, and 

 even a considerable portion of the palaeozoic periods — and if 

 we take account of the vast number of extinct species, 

 genera, and families needful to complete the various lines 

 of descent from the earliest known forms, presenting the 

 same numerical relations to those now living — we shall 

 be able to form some conception, however inadequate, of 

 the overwhelming frequency and importance of variations 

 in the size, form, proportions, and structure of the various 

 parts and organs of the higher animals, as compared with 

 variations in their number. No doubt, in the earlier 

 stages of organic development, numerical variations were 

 more frequent and more important, as they are now 

 among the lower forms of life, but at a very early period in 

 geological history, the main numerical relations of the 

 essential parts of the higher organisms became more or 

 less fixed and stable, and have in many cases remained 



