354 PROVISIONAL HYPOTHESIS Chap. XXVII 



sexually and asexnally, which, is very general. The former 

 pass in the course of their development from a very low stage 

 to their highest stage, as we see in the metamorphoses of insects 

 and of many other animals, and in the concealed metamorphoses 

 of the vertebrata. ■ Animals propagated asexually by buds 

 or fission, on the other hand, commence their development at 

 that stage at which the budding or self-dividing animal may 

 happen to be, and therefore do not pass through some of the 

 lower developmental stages.^^ Afterwards, they often advance 

 in organisation, as we see in the many cases of " alternate 

 generation." In thus speaking of alternate generation, I 

 follow those naturalists who look at this process as essentially 

 one of internal budding or of fissijiarous generation. Some 

 of the lower plants, however, such as mosses and certain alg{«, 

 according to Dr. L. Eadlkofer,^^ when propagated asexually, do 

 undergo a retrogressive metamorphosis. As far as the final 

 cause is concerned, we can to a certain extent understand why 

 beings propagated by buds should not pass through all the 

 early stages of development; for with each organism the 

 structure acquired at each stage must be adapted to its 

 peculiar habits ; and if there are places for the support of many 

 individuals at some one stage, the simplest plan will be that 

 they should be multiplied at this stage, and not that they 

 should first retrograde in their development to an earlier or 

 simpler structure, which might not be fitted for the then 

 surrounding conditions. 



From the several foregoing considerations we may conclude 

 that the difference between sexual and asexual generation 

 is not nearly so great as at first appears ; the chief difference 

 being that an ovule cannot continue to live and to be fully 

 developed unless it unites with the male element ; but even 

 this difference is far from invariable, as shown by the many 

 cases of parthenogenesis. We are therefore naturally led to 

 inquire what the final cause can be of the necessity in 



'° Prof. AUman speaks ('Transact. " of zooids, that no retrogression ever 



R. Soc. of Edinburgh,' vol. xxvi., 1870, " talces place in the series." 



p. 102) decisively on this head with ^^ 'Annals and Mag. of Nat. Hist., 



respect to the Hydroida : he says, "It 2nd series, vol. xx., 1857, pp. locJ- 



" is a universal law in the succession 455 



