ANIMAL PEDIGREES 219 



history of which entire chapters are lost, while in 

 those that remain many pages are misplaced and 

 others are so blurred as to be illegible ; words, 

 sentences, or entire paragraphs are omitted, and 

 worse still alterations or spurious additions have 

 been freely introduced by later hands, and at 

 times so cunningly as to defy detection. 



Very slight consideration will show that develop- 

 ment cannot in all cases be strictly a recapitulation 

 of ancestral stages. It is well known that closely 

 allied animals may differ markedly in their mode of 

 development. The common frog is at first a tadpole, 

 breathing by gills, a stage which is entirely omitted 

 by the West Indian Hylodes. A crayfish, a lobster, 

 and a prawn are allied animals, yet they leave the 

 egg in totally different forms. Some developmental 

 stages, as the pupa condition of insects, or the 

 stage in the development of a dogfish in which the 

 oesophagus is imperforate, cannot possibly be 

 ancestral stages. Or again, a chick embryo of, say 

 the fourth day, is clearly not an animal capable of 

 independent existence and therefore cannot cor- 

 rectly represent any ancestral condition, an objection 

 which applies to the developmental history of many, 

 perhaps of most, animals. 



Haeckel long ago urged the necessity of distin- 

 guishing in actual development between those 

 characters which are really historical and inherited 

 and those which are acquired or spurious additions 

 to the record. The former he termed palingenetic 

 or ancestral characters, the latter cenogenetic or 

 acquired. The distinction is undoubtedly a true 



