92 HOW ANIMALS DEVELOP 



aquatic. The tadpoles swim about freely, but the 

 adults are fixed to the sea-bottom and are simple 

 sacks, rather Hke sea-anemones in some ways. 

 Nearly all trace of their original structure is lost, and 

 we should hardly have reahzed that they are near 

 relations of the vertebrates if we did not know what 

 the larvae are like. 



It is not very obvious why the life-histories of 

 some animals should be compUcated by the develop- 

 ment of larvae forms when the animals are finally 

 going to turn round and become something abso- 

 lutely different. To develop into a butterfly by first 

 becoming a caterpillar is rather like going to Bir- 

 mingham by way of Beachy Head, as Chesterton has 

 it. Probably it serves a useful purpose by making the 

 embryo able to support itself before its development 

 is complete, so that there is no need to store so much 

 yolk in the egg to keep the embryo alive. But to show 

 why something is useful does not explain how it 

 arose. The evolution of complicated and specialized 

 larvae may be a consequence of the competition 

 between different young embryos for the food which 

 is suitable to young as opposed to adult creatures. As 

 soon as embryos start to feed themselves, they will 

 be submitted to the struggle for existence and must 

 prove themselves efficient or perish. Just as the same 

 sort of competition led to the evolution of the adults, 

 so it would force the embryos or larvae to become 

 more specialized and complicated. 



