94 Dr. Burnett on the Development of Viviparous Aphides. 



liar. With the Aphides there is no real morphological progress 

 made in each brood, for the viviparous individuals are, zoologically, 

 as perfect in every way as those which are oviparous, except in 

 their want of true sexual generative organs. I 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 oeconomical, 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 involves no 

 conditions excepting those belonging to a modified form of gem- 

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

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

 riations in manifestation would belong rather to the oecono- 

 mical relations of the animal, than to any intrinsic difference of 

 physiological process. Thus the Distoma-nurses, instead of being 

 developed to a condition resembling at all their parent, remain 

 persistent on a low form, and not only is their whole zoological 

 character undeveloped, but they also experience morphological 

 changes from the developmental process which immediately go on 

 within them. All this is in perfect keeping with their oeconomy 

 as animals, for the low order of their conditions of life does not 

 necessitate a higher and move truly zoological form of these nurses 

 from which are to be developed the true animals; were it other- 

 wise, 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 oeconomical conditions are dif- 

 ferent, and finely 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, 

 zoologically, proceeds to a corresponding point ; this point is 

 where it, as an insect and as an Aphis, can furnish the nutritive 

 material for the development of its endogenous germs. 



Herein, then, would appear to consist the prominent morpho- 

 logical differences observed in this category of phenomena, and 

 I need not labour further to show that they are irrelevant of the 

 primary essential conditions of these curious processes. 



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

 zoological, interpretation that can be advanced for these pheno- 

 mena which Steenstrup has so ingeniously collected and collated j 

 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 — to put forth this view, I say, is to advo- 

 cate a doctrine in physiology as mischievous as it is deeply 



