INFANCY OF CRABS AND CATERPILLARS 243 



" Arthropoda," the outer skin of the body cannot increase, 

 and hence every now and then the growing youngster has 

 to wriggle out of its skin and form a new one. More 

 correctly, the new skin forms under the old, so that when 

 the latter is shed all that remains is to harden the new 

 integument and begin to feed again. 



So far so good. Now let us survey these caterpillars a 

 little more closely. To begin with, they are infant butter- 

 flies or moths, as the case may be. But for them there are 

 no parental ties ; and we may be certain that the parents 

 cannot recognise their own offspring. Commonly, indeed, 

 they die long before their young launch themselves upon 

 the world. The two phases of life are absolutely different 

 and distinct. And here, more easily perhaps than any- 

 where else in the animal kingdom, we can see how the 

 environment acts upon the individual — how the larval 

 stage is moulded and modified by its present environment — 

 and thereby we shall the more readily understand how it is 

 that the larvas of other animals may not only represent 

 lowlier, ancestral conditions of bodily structure, but also 

 how large is the element of probability that we may not 

 seldom fail to distinguish between ancestral phases in 

 larval life and phases which are demanded by the imme- 

 diate, present environment, as the minimum standard of 

 response to the demands necessary for the survival of the 

 race to-day. 



Let us make this clear. In an earlier chapter it was 

 shown that the tail of young fishes passes through the 

 ancestral stages before arriving at the form characteristic 

 of the adult stage, which is of a quite different form — e.g. 

 the young Angler. The larvae of many fishes, and most 

 frogs, for some time breathe by external gills : these may 

 be ancestral adult, or ancestral larval conditions ; but that 



