30 INTRODUCTION. 



Causes tending to falsify the ancestral history ; or to prevent 

 ontogeny from being a true record of phylogeny. 



1. The general tendency to condensation of the ancestral 

 history. Except perhaps in the lowest groups of Metazoa, such 

 as sponges, no animal can possibly repeat, in its own develop- 

 ment, all the ancestral stages in the history of the race. There 

 is a tendency in all animals towards striking a direct path from 

 the egg to the adult : a tendency best marked in the higher, the 

 more complicated members of a group, i.e. those which have 

 the longest and most tortuous pedigrees. 



2. The tendency to the omission of ancestral stages. This 

 has been already noticed as one of the commonest effects of 

 abundance of food-yolk. The omission of the gill-breathing 

 stage in Hylodes and in all Amniote Vertebrates is a typical 

 example. 



3. The tendency to distortion, either in time or space. All 

 embryologists have noticed the tendency to anticipation, or pre- 

 cocious development, of characters which really belong to a later 

 stage in the pedigree. Many early larvae show it markedly, 

 the explanation in this case being that it is essential for them 

 to possess at the time of hatching all the organs necessary for 

 independent existence. 



Anachronisms, or actual reversals of the historical order of 

 development of organs or parts, occur frequently. Thus the 

 joint surfaces of bones acquire their characteristic curvatures 

 before movement of one part on another is effected, and even 

 before the joint cavities are formed. 



4. The tendency to the accentuation or undue prolongation 

 of certain stages. This is best seen in cases of abrupt metamor- 

 phosis, as of the caterpillar to the butterfly ; or of the pelagic 

 pluteus larva to the sea urchin, slowly crawling on the sea- 

 bottom ; or of the herbivorous aquatic tadpole to the terrestrial 

 and carnivorous frog. In such cases there is usually a great differ- 

 ence between larva and adult in external form and appearance, 

 in manner of life, and very usually in mode of nutrition ; and a 

 gradual transition is inadmissible, because in the intermediate 

 stages the animal would be adapted neither to the larval nor to 

 the adult conditions ; a gradual conversion of the biting mouth 

 parts of the caterpillar to the sucking proboscis of the moth would 



