^^^^ ZOOGENESIS "^""^ 



and the branches do not join the main trunk; and 

 besides, the main trunk itself is not continuous. The 

 gap between the very earliest horses that we know, 

 the horses of the Eocene, and those of the next fol- 

 lowing period, the Oligocene, will undoubtedly be 

 narrowed by future discoveries, but most of the gaps 

 probably will not. 



Especially is it true that evolutionary lines tend to 

 be separated from each other for their entire course, 

 the trunks of the evolutionary trees running parallel 

 or more or less convergent right down to their very 

 earliest beginnings and not coming together in a 

 common type of animal as we would expect. 



For instance, the whale line is always distinct from 

 every other line of mammalian development, just as 

 in the reptiles the corresponding ichthyosaurs always 

 were distinct from every other line of reptiles. So 

 also the seal line is always distinct. Just as whales 

 were always whales, seals were always seals. No 

 intergrades between the seals and other mammals are 

 known, although the seals belong to the Carnivora 

 and therefore must have had an ancestor common to 

 them and to the terrestrial members of that great 

 mammalian group. 



Among the more familiar mammals, the cat and 

 the dog lines are always separate. No forms inter- 

 mediate between cats and their relatives and dogs and 

 their relatives are known, even though both cats and 

 dogs are collateral members, together with the seals, 

 of the Carnivora. 



But in the case of cats and dogs the very early types 



[TsT] 



