FROM EMBRYOLOGICAL DATA. 21 



details, as soon as from these generalizations we begin to investigate the peculiar- 

 ities of each type, then let us be guided by special principles, and no longer by the 

 general abstract laws of life. 



Now, in the type of Articulata, we have division of the body, articulation of the 

 limbs, development of joints, external evolution of parts, and multiplication and inde- 

 pendence of these parts, prominent in every type under most diversified modifica- 

 tions ; and these peculiarities characterize the type of Articulata. External meta- 

 morphosis is, perhaps, on the whole, the most prominent feature of this type ; that is, 

 extensive changes in the external appearances of the body, changes which follow 

 in regular succession at definite periods of life, and introduce successively such 

 modifications in some of these animals, in which the metamorphosis is most exten- 

 sive, as completely to alter their appearance ; so much so, that only direct observation 

 of successive generations can satisfy us that, under these different forms, we have 

 really the different stages of growth of one and the same animal. The study of 

 these changes of form is the more important,' as the structure changes at the same 

 time ; for here changes of the form and of the structure are so closely related as to 

 be simultaneous. 



Now it is a principle which must be acknowledged universally, that in the growth 

 of animals their successive changes are progressive, are steps forwards, an ad- 

 vance from a lower to a higher condition. However evident the decrepitude of old 

 age may be in the structure of an animal, as long, at least, as it grows and succes- 

 sively assumes new changes, we cannot for a moment suspect that these changes are 

 retrograde steps in the organization. This must be granted even in the presence of 

 the deformed appearance and extreme growth of pregnant individuals, or of females 

 preparing for reproduction, among parasitic Crustacea. For here we observe a kind 

 of metamorphosis which has no longer a direct bearing upon the growth of the 

 individual, but has reference to its reproduction. I insist upon this point, as other- 

 wise the principle of progressive development might be questioned even in its fun- 

 damental consequences. 



But as soon as it is granted that the young animal is less perfect than the mature 

 one, as can be shown by tracing its development from the egg, it will follow as a 

 matter of course that the metamorphoses of Articulata, considered in general, must 

 be the safest guide to a just appreciation of the relative position of the secondary 

 groups in the whole type. The most perfect metamorphoses are known among In- 

 sects. They appear in their caterpillar, pupa, and perfect state. But these meta- 

 morphoses have various degrees in different families. The larvae of all Insects are 

 not equally advanced when they are hatched from the egg. In some, the larva comes 

 out as a naked, colorless worm, without any appendages ; in others, it has simple legs 

 of one kind ; in others, there are various sorts of legs, and the body is variously col- 

 ored and provided with external appendages ; in others, the larval development takes 

 place altogether within the egg, and the young insect is hatched in the form of the 

 parent, wanting only wings. But in all these different conditions the larva is, 

 undoubtedly, inferior to the pupa. The pupa state of Insects is the period of their 

 development which, at the present time, stands most in need of further investiga- 



