246 



CHAPTERS ON EVOLUTION. 



preceding cases of altered development, consists in the observation 

 that the Mexican axolotl, apparently a mature form, was able to 

 reproduce its species. It may perhaps prove a truer conception of 

 the case if we regard the axolotl as a "permanent larval form," 

 which has acquired the power of producing young, and which has 

 therefore assumed the form, life, and constitution of a species. 

 Analogy supplies us with a valuable series of parallel instances from 

 the records of natural history science. The example in question, of 

 a larva acquiring reproductive powers, is by no means singular or 

 unique. We have seen that practically a pteropod or " Sea butterfly " 

 (Fig. 152, B) is essentially the larval form of the gasteropod (A), 

 which has had its immature character fixed, and which has acquired 

 the power of producing young. Other cases of this peculiarity are 

 readily found within the confines of the insect class, and 

 in other divisions of the animal world. Thus, we know 

 that the larva or maggot itself an absolutely immature 

 form of a fly (Cecidomyia) (Fig. 164), produces other 

 larva like itself, and these in turn produce others (a, #), 

 which, finally becoming males and females, produce 

 normal young through eggs. There is another insect 

 (Chironomus) of which, as Grimm has shown, the 

 chrysalis lays eggs ; and we know of cases in which 

 (as illustrated by the Aphides or plant-lice, and by the 

 queen bees) perfect young may be produced by the one 

 insect alone. So likewise the common Newt (Triton 

 cristatus] of our ponds may, occasionally, when imma- 

 ture, produce young ; and another species ( T. Alpestris) 

 has been seen to reproduce its kind when it was still in 

 the tadpole stage. Amongst the zoophytes, such features 

 are still more plainly marked. For a plant-like animal 

 colony gives origin to jellyfishes, which swim freely in 

 CEciDolfviA. the sea, and later on produce eggs, from each of which a 

 zoophyte in turn springs. These facts were formerly 

 included under the head of " alternation of generations ; " but under 

 whatever name we denote the phenomena, the lesson they teach is 

 uniform and clear. Such cases as these of the insect larva and the im- 

 mature axolotls and new s producing young (contrary to the rule that 

 only adult animals reproduce their species) prove to us that, if condi- 

 tions be favourable, a young animal's development and constitution 

 may be so modified and intensified, that it may, whilst still under its 

 larval guise, produce young, and thus assume the likeness and functions 

 of a new and distinct species. Such facts further impress the idea that 

 the young being, equally with the adult, is liable to modification and 

 change ; and they therefore teach us that the starting points of new 

 species and races do not always lie within the domain of mature 



