K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 229 



is at least separated from the animal; but in Insects with 

 metamorphosis, sensibility, irritability and plasticity are sepa- 

 rated, though indeed only in the perfect state. The most highly 

 developed among these, again, appear to me to be those whose 

 thoracic segments are not divided into many separate rings, 

 as in the Flea, the Coleoptera, and the Orthoptera, but when 

 they are collected into one. It is in these that the originally 

 similar parts, such as the feet and manducatory organs, have at- 

 tained the widest deviation. It is in these that we meet with 

 the best developed wings and the most various manifestations of 

 life. The Crustacea, indeed, possess an ear and a nose ; but we 

 must not forget that the head of Insects is small enough to 

 make such organs doubtful ; that a few investigators believe 

 they have found them, and that, at any rate, the senses are not 

 absent. 



If we have succeeded in getting rid of all preconceived notions 

 of a gradual series, we shall consider each form to be a modifi- 

 cation of a more general form, and that these are modifications 

 of a fundamental type, and learn to comprehend them from this 

 point of view. We shall then take more pains to determine 

 the general affinities of each species, than its place in a univer- 

 sal progressive series. 



If we seek, however, for the grade of development, we must 

 look for it only according to the degree of differentiation of the 

 parts, and within the type which belongs to the animal. That, 

 however, in fact, the preconceived conceptions of a progressive 

 series have led to the ordinary views, is what I shall endeavour 

 to establish and explain by yet a few examples. 



We often hear of retrogressive metamorphoses of a whole 

 animal form or of a single organ ; can any clear conception 

 attach to these " Retrogressions/ 5 if we do not assume that the 

 form of some one animal is the condition of the form of some 

 other? This much, however, is certain, that such a view implies 

 the conception of a serial progression. 



If, in fact, we arrange together obviously allied animals, and 

 then place them with their highest forms below those of another 

 series, we shall have a retrogression. I will but shortly refer to 

 the above (Schol. V. 3 a) example of Fishes. 



In truth, the retrogressive metamorphosis of particular organs 



