78 ZOOLOGY 



yolk. The limbs of a larva must be functional those of an 

 embryo are often functionless stumps. Then again in both 

 phases of life-history, but most markedly in the embryonic, 

 we find a tendency towards the dislocation of development, 

 by which phrase is meant the precocious development of 

 organs which are of importance to the adult, so that the 

 body of an embryo at one time may represent two periods 

 of ancestral history at the same time, each affecting a 

 different group of organs. This indeed seems to be one 

 of the most frequent factors in modifying development. 

 Lastly, the circumstances of the larva may undergo secondary 

 changes whilst those of the adult remain relatively the same : 

 in this case special larval modifications may arise which may 

 obscure the ancestral type; thus in the majority of cases 

 insect-larvae live in very special surroundings to which they 

 are specially adapted. 



If we then accept the theory of recapitulation as having the 

 great balance of probability in its favour, we may now reflect 

 on what this theory implies as to the course which has been 

 pursued by evolution. According to this theory a larva 

 primarily represents an ancestral stage in the history of the 

 species, and the larval circumstances are a picture of the 

 ancestral environment. When the larva changes into the 

 adult, or to use the technical term, metamorphoses, it deserts 

 the ancestral or larval environment and enters into the modern 

 and adult one. We seem therefore driven to conclude that in 

 past time a new species was made out of an old one by the 

 migration of some of its members into a new environment 

 and their consequent exposure to new influences amongst which 

 the most potent may have been a new kind of food. Now in 

 Chapter II it has been pointed out that every species tends to 

 reproduce itself at such a rate that if unchecked it would soon 

 overrun the world : it was indeed compared to a fire in a dry 

 prairie, which is always striving to spread at its borders. This 

 comparison appears to be increasingly justified the more the 

 circumstances of the life of species is looked into. It has been 

 shown that for many species there is a mother country, in 

 other words, a suitable environment where each flourishes and 

 multiplies so that in this country it is swarming. The surplus 

 population spreads outwards from the borders of the country 



