THE PERPETUATION OF LIFE 337 



In the early stages of development animals in general 

 are much more alike than they are later. As they develop 

 the embryos of different groups become more and more 

 dissimilar; those of distantly related groups are strikingly 

 different in their early stages, while the embryos of closely 

 related forms usually show a strong resemblance in all 

 periods of development. The embryos of higher animals 

 are often similar to the adult stages of animals which stand 

 below them in the scale of life. These similarities, as 

 we shall see in a later section, point clearly to a descent 

 of the higher animals from more primitive forms. 



Some animals when hatched or born bear a fairly close 

 resemblance to the adult condition, but in many others 

 the young as they first emerge are so different from their 

 parents as to appear to belong to a quite different group 

 of animals. Such animals are said to undergo a metamor- 

 phosis in developing into the mature form. Instances are 

 furnished by the butterflies and moths, beetles, flies and 

 many other insects, by the transformation of tadpoles 

 into frogs, and by the larval stages of many marine 

 invertebrates. In several cases larvae have been described 

 as new types of life before it was discovered that they 

 represented the young stage of some previously known 

 animal. 



In higher animals a part of the function of perpetuating 

 life consists in caring for offspring until they are able 

 to shift for themselves. Among low forms the young 

 receive no attention whatever from their parents, most 

 of whom do not recognize their offspring as their own. 

 The young spider is a nimble, active creature, that can 

 spin its own web practically as well as in later life. Many 

 Crustacea carry their eggs and for a time their young in 

 brood pouches or attached to appendages of the abdomen, 

 but they never care for their offspring in any way, and 



