328 E. W. MACBBIDE. 



portion of the development would become embryonic, and so 

 subjected to the various modifying influences which are con- 

 nected with this type of ontogeny. Therefore it follows, as 

 the first important deduction from Sedgwick^s theory, that in 

 seeking to obtain a basis for phylogeny, most importance must 

 be attached to animals which show long larval histories. 



Balfour with his usual sagacity has, so to speak, instinctively 

 anticipated this conclusion. Although he points out that " the 

 favourable variations which may occur in the free larva are 

 much less limited than those which can occur in the fcetus," 

 he says that there is " a powerful counterbalancing influence 

 tending toward the preservation of ancestral characters, in that 

 larvae are compelled at all stages of their growth to retain in a 

 functional state such systems of organs at any rate as are 

 essential for a free and independent existence " {' Comp. Emb.,' 

 vol. ii, p. 299). 



The objection, alluded to in Balfour's statement, that larvae 

 as well as adults have been subjected to the modifying influ- 

 ences of their environment, will readily occur to most minds. 

 Let us consider whether it is possible to approximately estimate 

 the nature and amount of such influences ; and, first of all, let 

 us consider what is meant by secondary larvae. 



Balfour imagined that secondary larval forms might be pro- 

 duced by a diminution in the food yolk, and consequent earlier 

 commencement of free existence (loc. cit., p. 300). There is 

 no evidence to suggest that such a change has ever taken 

 place ; all the facts point in a contrary direction. We shall 

 see that food yolk produces the most diverse distortions of 

 development; the developmental processes of free larvae are, 

 on the other hand, remarkably uniform. Secondary larvae 

 must be regarded as having arisen owing to the young adults 

 having taken to a new mode of life ; the best instances of this 

 are perhaps the aquatic larvae of the may-flies and dragon-flies. 

 We have the strongest reason for believing that the immediate 

 ancestors of insects were terrestrial animals, and the aquatic 

 larvae mentioned show their secondary character by the fact 

 that their respiratory organs are modified from organs adapted 



