1 2 PROFESSOR A. M. MARSHALL. 



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 development 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 develop- 

 ment. The common frog is at first a tadpole, breathing by gills, 

 a stage which is entirely omitted by the West Indian Hylodes. A cray 

 fish, 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 correctly 

 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 distinguishing 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 one, but 

 an exceedingly difficult one to draw in practice. The causes which 

 prevent development from being a strict recapitulation of ancestral 

 characters, the mode in which these came about, and the influence 

 which they respectively exert, are matters which are greatly exercising 

 embryologists, and the attempt to determine which has as yet met with 

 only partial success. 



The most potent and the most widely spread of these disturbing 

 causes arise from the necessity of supplying the embryo with 

 nutriment. This acts in two ways. If the amount of nutritive 

 matter within the egg is small, then the young animal must hatch 

 early, and in a condition in which it is able to obtain food for itself. 

 In such cases there is of necessity a long period of larval life, during 

 which natural selection may act so as to introduce modifications of the 

 ancestral history, spurious additions to the text. 



If, on the other hand, the egg contain within itself a considerable 

 quantity of nutrient matter, then the period of hatching can be post- 

 poned until this nutrient matter has been used up. The consequence 



