Dr. Burnett on the Development of Viviparous Aphides. 93 



ally, 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 Steefistrup* in his doctrine 

 of the "Alternation of Generations," and of which it forms a 

 chief support. The details of this peculiar doctrine of Steen- 

 strup I need not here furnish ; they are well known to all physio- 

 logical anatomists. Its features, however, may be expressed in a 

 formula-like manner. Individuals A produce true fecundated 

 eggs, from which are hatched individuals B, which are unlike 

 their parents in all zoological respects, but in which are developed 

 spontaneously and without any reference to sex, germs which ulti- 

 mately become individuals like A, and so the cycle of development 

 is completed. These intermediate individuals, B, Steenstrup has 

 termed nurses {Amnen), and he regards them as distinct animaU 

 subservient for a special end; he therefore considers that B 

 constitutes a real generation. 



Instances of such ph?enomena are found in the lower orders of 

 the animal kingdom — Polyps, Acalephs and Worms ; and late re- 

 search has shown that they are more or less common throughout 

 the whole of the Invertebrata. 



, The difference between alternation of generation and metamor-; 

 phosis 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 deve- 

 lopment. Another important 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 

 examples known of nursing individuals, and, on the whole, as con- 

 stituting typical illustrations of the doctrine he has advanced f. 



Bnt 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 pha^nomena in question of the Aphides. And 

 although I am inclined to believe, as I shall soon show, that all 

 these phsenomena, essentially, may be of the same nature, yet there 

 can be no doubt that the manifestations are here somewhat pecu- 



* On the Alternation of Generations, or the Propagation and Develop- 

 ment of Animals through Alternate Generations ; a peculiar form of fostering 

 the young in the lower classes of Animals. Transl. by the Ray Society. 

 London, 1845, passim. 



t See Steenstrup, loc. cit. p. 112. 



