SPECULATIVE ZOOLOGY. 375 



may be added. A young form may become adapted to a new mode 

 of life, or it may escape competition by seizing upon a new field ; its 

 enemies, dangers, and means of defense may change, and with these 

 changes of habits corresponding changes of structure may occur, so 

 that the primitive or ancestral record may become completely obscured 

 by secondary changes. 



The examination of the various kinds of modification which may 

 be brought about in this way falls within the scope of a treatise on 

 comparative embryology, but it would be out of place here, although 

 one or two examples of the more common sorts of modification may 

 be of interest. 



The chrysalis stage of butterflies is an instance of the secondary 

 acquisition of a new stage of development, which forms no part of the 

 ancestral record. 



It is obvious that the inactive pupa, which takes no food, and 

 exhibits few of the ordinary activities of animal life, can not pos- 

 sibly have existed in the past as an adult ancestor of the butter- 

 flies, nor is it conceivable that any of the remote ancestry of this 

 group bore a general resemblance to a pupa. While it is impossible 

 to believe that the pupa stage is ancestral, we have good evidence to 

 show the manner in which it has been acquired as a secondary modi- 

 fication. 



Lubbock has pointed out that the least specialized or most primi- 

 tive insects have mouth -parts which are indifferently adapted for 

 either cutting or sucking, and that these insects do not undergo a 

 metamorphosis, but are gradually converted into the adult form by a 

 simple process of gradual development. He also shows good ground 

 for believing that the common ancestors of all the groups of insects 

 were like these forms in these particulars ; and he holds that, as the 

 stock which led to our present butterflies was evolved from this 

 ancient stem-form, the young became adapted to a sedentary creep- 

 ing life, and their indifferent mouth-parts became gradually converted 

 into cutting jaws, while the adults became adapted to quite a different 

 mode of life, and the same indifferent mouth-parts became gradually 

 modified into a sucking proboscis. While the caterpillar and butter- 

 fly were thus diverging in two directions from the original unspecial- 

 ized form, and the structure and habits of the larva were becoming 

 more and more different from those of the adult, it is plain that the 

 metamorphosis must at the same time have become more and more 

 violent ; and, according to Lubbock, one of the periods of slight ac- 

 tivity which, in most insects, accompany the periodical molts, was 

 seized upon, and gradually extended into a long resting or chrysalis 

 stage, in order to enable the animal to exist while the highly special- 

 ized organs of the caterpillar are changing into the equally specialized 

 but very different organs of the winged insect. The growing but- 

 terfly now passes through a resting or pupa stage which connects tho 



