io THE ART OF MORPHOLOGY 



type may be quoted as instances of the second 

 order of magnitude ; the hoofed mammal or 

 ungulate type and the primate type are of the 

 third order ; the odd-toed ungulate of the fourth 

 order ; the equine type and the human type are 

 examples of the fifth order of magnitude. Lower 

 categories are exemplified by the type genus of 

 a family, the type species of a genus, and the 

 type individual of a species, the last being purely 

 conventional, referring to the actual specimen 

 upon which the original specific description was 

 based. The types of cultivated races are deter- 

 mined in a somewhat different manner. 



From the fourth grade downwards it is 

 frequently possible to determine the pedigree 

 of a specialised type from a more generalised 

 ancestry, with considerable probability of accuracy 

 and, in some cases, as with the pig, horse, and 

 elephant, with a great deal of precision. In the 

 superior grades it becomes increasingly difficult 

 to arrive at an understanding. It is something 

 to be able to say that the vertebrate type can 

 be traced with certainty to a chordate type ; 

 but it cannot be asserted without contradiction 

 that the craniate vertebrate is derived from an 

 acraniate chordate ; because the gap between 

 these two grades of organisation is too great, 

 and many of the intervening stages of substitu- 

 tion and of cephalisation (head - formation) are 

 lost. 



