74 Dr. Burnett on the Development of Viviparous Aphides. 
of the “Alternation of Generations,” and of which it formsa 
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 ul- 
timately become individuals like A, and so the cycle of develop- 
ment is completed. ‘These intermediate individuals, B, Steen- 
strup has termed nurses (Ammen), and he regards them as distinct 
animals subservient fora 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 re- 
search has shown that they are more or less common throughout 
the whole of the Invertebrata. 
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 this doctrine he has advanced.* 
first to the last, inclusive, is merely a repetition of the same. 
But these conditions are external and economical, and, instead of 
* See, Steenstrup, loc. cit., p, 112. 
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