326 E. W. MAOBRIDE. 



is embryonic, but in every case larval development is preceded 

 by a longer or shorter period of embryonic development. 



The whole interest of the science of Embryology lies, of 

 course, in the fact that features observed in both types of de- 

 velopment seem inexplicable except on the assumption that 

 they are reminiscences of structures possessed by the ances- 

 tors of the animals in whose development they appear. Such 

 traces of the history of the race are to be found in the vast 

 majority of larva? ; in embryos they are likewise to be found, 

 though here they are less prominent, as is seen by comparing 

 the development of two allied forms, in one of which the larval 

 type prevails, and in the other the embryonic. Now Mr. Sedg- 

 wick's theory of the relation of the two types to one another is 

 that that portion of embryonic development in which ancestral 

 features are observable represents a larval stage passed over 

 inside the uterus or egg-membrane and modified in con- 

 sequence. Thus the chick during the first four or five days 

 of its existence is to be regarded as an immensely modified 

 larva. 



If this view be true it follows that, however modified the 

 record of ancestral history contained in the larval development 

 may be, the embryonic record of the same history can never 

 rise above it in value. 



It was until lately customary to assume, explicitly or im- 

 plicitly, that there was an inherent tendency for the ontogeny 

 of the individual to be a summarised repetition of the phylo- 

 geny of the race. In proof of this statement we may adduce 

 Balfour, who in his ' Text-book of Comparative Embryology' 

 (vol. ii, p. 298), says, ^' Unless secondary changes intervened 

 this record [of ancestral history] would be complete;" and 

 Bateson,^ in his discussion of the ancestry of the Chordata, 

 commits himself to a similar position. That there can be no 

 such general tendency is, however, shown by the fact that in 



' "Tlie Ancestry of the Chordata," W. Bateson, 'Quart. Journ. Micr. 

 Sci.,' 1886. "Development within an egg-shell as involving a less compli- 

 cated struggle with enviromental forces, is less subject to variation than that 

 in the open sea, and consequently is more likely to preserve ancestral features." 



