196 LAKE SUPERIOR. 



giving us another guide to the manifold relations which exist among 

 animals, allowing us to avail ourselves, for the purpose of classification, 

 of the facts derived from the development of the whole animal king- 

 dom in geological epochs, as well as the development of individual 

 species in our epoch. But to this most fruitful principle I shall have 

 hereafter an opportunity of again calling attention. 



At present there is some doubt among zoologists, as to the respect- 

 ive position of the classes of worms, insects and Crustacea, some 

 placing the Crustacea, and others the insects uppermost. Embryonic 

 data may afford the means of settling this question ; we need only 

 remember the extensive external changes which insects undergo from 

 their earliest age, and the many stages of structure through which 

 they pass, whilst Crustacea are less polymorphous during the different 

 periods of their life, and never obtain an aerial respiration, but 

 breathe through life with gills, which many larvse of insects cast before 

 they have accomplished their metamorphoses, to be satisfied that the 

 afiinity between Crustacea and worms is greater than between worms 

 and insects, especially if we consider the extraordinary forms of some 

 parasitic types of the former. As soon as the higher rank of insects 

 among Articulate is acknowledged, many important relations, which 

 remain otherwise concealed, are at once brought out. The whole 

 type of insects in its perfect condition, contains only aerial animals, 

 while the Crustacea and worms are chiefly aquatic. And if we com- 

 pare these three classes in a general way, we cannot deny the cor- 

 rectness of the comparison as made by Oken, that worms corres- 

 pond to the larval state of insects, Crustacea to their pupa state, 

 and that insects pass through metamorphoses corresponding to 

 the other classes of Articulata. The little we know about the 

 embryology of worms will already satisfy us that the earlier 

 stages of the higher of these animals agree most remarkably in 

 character with such of them as, from other reasons, we have been 

 in the habit of considering as the lowest, thus affording another 

 prospect of regulating finally the arrangement of those curious 

 animals entirely upon embryonic data. 



If there is any internal evidence that the whole animal kingdom is 

 constructed upon a definite plan, we may find it in the remarkable 



