54 



GENERAL EVOLUTION". 



act is a more frequent relation than the 

 exact, while the more complex inexact 

 relation is still more common. The 

 greater the inexactitude, the more fre- 

 quently do such parallels occur till we 

 have those of the most remote character, 

 as, for instance, the parallelism between 

 the different stages of the development 

 of the mammal, in the structure of the 

 heart and origins of the aorta, and the 

 existing classes of vertebrates. The re- 

 lation of these facts to the origin of 

 genera will be noted hereafter. 



It will be borne in mind that in the 

 Scaphiopodidae the generic types are 

 identical for a long portion of their 

 developmental history. (See figures on 

 plate iv for the representation of these 

 facts. ) 



2. In both Perissodactylous and Ar- 

 tiodactylous Mammalia, certain types 

 develop their family character of ca- 

 nines at the earliest appearance of den- 

 tition, others not till a comparatively 

 late period of life (Equus), and the ex- 

 treme individuals never produce them. 



3. Among cetaceans the genus Orca 

 maintains a powerful and permanent 

 series of teeth, which is an important 

 generic character. In Beluga the se- 

 ries is shed in old age, in Globiocepha- 

 lus, or the Caing whales, they are shed 

 at middle age, while in the Balsenidse, 

 of which the absence of teeth is an es- 

 sential character, these organs are de- 

 veloped and absorbed during fcetal life 

 (Eschricht). Though the condition 

 of the teeth is not of systematic value 

 in the two named intermediate genera, 

 it is the important feature in the his- 

 tory of progress to such value. 



