4498 Insects. 



of Generations," so called, or in the conditions of true gemmiparity, 

 — admitting, provisionally, that Steenstrup's doctrine and gemmiparity 

 include really different physiological conditions. 



But the most important explanation advanced, and the last which 

 I shall notice, is that offered by Steenstrup in his doctrine of the 

 " Alternation of Generations," and of which it forms a chief support. 

 The details of this peculiar doctrine of Steenstrup I need not here 

 furnish; they are well known to all physiological anatomists. Its 

 features, however, may be expressed in a formula-like manner. Indi- 

 viduals A produce true fecundated eggs, from which are hatched indi- 

 viduals B, which are unlike their parents in all zoological respects, 

 but in which are developed spontaneously, and without any reference 

 to sex, germs which ultimately become individuals like A, and so the 

 cycle of development is completed. These intermediate individuals, 

 B, Steenstrup has termed nurses (Am?ien), and he regards them as 

 distinct animals subservient for a special end ; he therefore considers 

 that B constitutes a real generation. 



Instances of such phenomena are found in the lower orders of the 

 animal kingdom — Polyps, Acalephs and Worms ; and late research 

 has shown that they are more or less common throughout the whole 

 of the Invertebrata. 



The difference between alternation of generation and metamorphosis 

 is too marked to require illustration : in the latter there is the same 

 individual throughout, and the developmental processes, although 

 concealed beneath different exteriors, are regular and normal ; with 

 the former, however, this chain of development is broken by one form 

 being developed in another, this intermediate form serving as a 

 stepping-stone for a higher and ulterior development. Another im- 

 portant point in this alternate reproduction is, that in each new 

 change some real progress is made, the nursing-form being manifestly 

 inferior to the individual to which it gives rise. 



Steenstrup regards the Aphides as furnishing the most perfect ex- 

 amples known of nursing individuals, and, on the whole, as consti- 

 tuting typical illustrations of the doctrine he has advanced. 



But if this doctrine implies conditions other than those which 

 belong to true gemmiparity, it does not appear to me that it has any 

 support in the phenomena in question of the Aphides. And although 

 I am inclined to believe, as I shall soon show, that all these pheno- 

 mena, essentially, may be of the same nature, yet there can be no 

 doubt that the manifestations are here somewhat peculiar. With the 

 Aphides there is real morphological progress made in each brood, for 



