Insects. 4499 



the viviparous individuals are, zoologically, as perfect in every way as 

 those which are oviparous, except in their want of true sexual gene- 

 rative organs. 1 have shown that, in the one species here described, 

 they had well-developed wings like the true sexual individuals. 

 Moreover, each brood, from the first to the last inclusive, is merely a 

 repetition of the same. But these conditions are external and econo- 

 mical, and, instead of offering these prominent examples as evidence 

 against the validity of Steenstrup's doctrine, I would rather present 

 them as broadly indicating that, after all, this doctrine in question in- 

 volves no conditions excepting those belonging to a modified form of 

 gemmiparity. All the instances of Polyps, Acalephs, Worms, Insects, 

 &c, would then be classed in the same category, and the variations in 

 manifestation would belong rather to the economical relations of the 

 animal than to any intrinsic difference of physiological process. Thus 

 the Distoma-nurses, instead of being developed to a condition resem- 

 bling at all their parent, remain persistent on a low 7 form ; and not 

 only is their whole zoological character undeveloped, but they also 

 experience morphological changes from the developmental process 

 which immediately goes on within them. All this is in perfect keeping 

 with their economy as animals, for the low order of their conditions 

 of life does not necessitate a higher and more truly zoological form of 

 these nurses from which are to be developed the true animals : were 

 it otherwise, I cannot but believe that both the nurses and the grand 

 nurse of Distoma would quite resemble the original animals. In the 

 case of the Aphides, the economical conditions are different, and 

 finally illustrate this point. 



The Aphis-nurse, in virtue of its very typical structure as an insect, 

 must live under higher conditions, and so its development, zoologi- 

 cally, proceeds to a corresponding point : this point is where it, as an 

 insect and as an Aphis, can furnish the nutritive material for the de- 

 velopment of its endogenous germs. 



Herein, then, would appear to consist the prominent morphological 

 differences observed in this category of phenomena, and I need not 

 labour further to show that they are irrelevant of the primary essen- 

 tial conditions of these curious processes. 



Such appears to me to be the highest, both physiological and 

 zoological, interpretation that can be advanced for these phenomena 

 which Steenstrup has so ingeniously collected and collated; and to 

 advance the view that these intermediate individuals or nurses are not 

 intrinsically and zoologically the same as their parents, but furnish 

 examples of how dissimilar animals may arise from a common stock, 



