INTRODUCTION. XX111 



or have they been evolved on the other side of the globe 

 through a distinct set of carnivorous ancestors ? The case of 

 the horses is often cited as sugge'sting that such a parallelism 

 in evolution may have occurred ; because the series of ancestral 

 horses traced through the Tertiary strata of Europe is closely 

 similar to, but not quite identical with the ancestral series 

 found in the same order in the corresponding rocks of North 

 America. Here the facts are tolerably well-known, but they 

 admit of more than one interpretation. An easy land-connection 

 between Europe and North America, throughout the Tertiary 

 period, if allowed by geological considerations, might account 

 for the phenomena observed even if all the horse-like animals 

 were evolved from one stock in one and the same area. 



Theory of Recapitulation. There is also the well-known and 

 widely-accepted principle that the stages in the development 

 of an individual organism at the present day repeat in a general 

 manner the successive phases through which the ancestors 

 of that organism have passed in former periods of the earth's 

 history. There is no doubt, for example, that in the course 

 of its individual development the homocercal tail of a modern 

 bony fish passes through the same stages as those successively 

 exhibited by the majority of the adult fishes at the different 

 geological epochs. It is also evident that the family of deer 

 (Cervidse) has gradually acquired complex antlers in precisely 

 the same manner as every modern stag acquires them during 

 the course of its individual life. Again, the " cloven foot "of 

 the existing ruminant appears in the embryo with separated 

 metapodial bones, like those of the adult ancestral ruminants. 

 It is also tolerably certain (though fossils have not yet provided 

 absolute demonstration) that the rudimentary teeth and hind 

 limbs of the existing whalebone whales (Mystacoceti) are in- 

 herited from functionally toothed quadrupedal ancestors. Em- 

 bryology, however, cannot afford much precise information 

 concerning these processes of evolution, because an embryo 

 usually exists under physiological conditions totally distinct 

 from those influencing an adult. The embryo exhibits features 

 derived from its ancestors (palingenetic characters) inextricably 

 mingled with features due to the peculiar circumstances under 



