162 THE STORY OF LIFE'S MECHANISM. 



This fossil record has given us our best know 

 ledge of the course by which the present living 

 world has been brought into its existing condi- 

 tion. But its accuracy is largely confined to the 

 recent periods. Of the very early history fossils 

 tell us little or nothing. All the early rocks, 

 which we may believe were formed during the 

 period when the first steps in this machine build- 

 ing were taken, have been so changed by heat 

 and pressure that whatever specimens they may 

 have originally contained have been crushed out 

 of shape. Furthermore, the earliest organisms 

 had no hard skeletons, and it was not until living 

 beings had developed far enough to have hard 

 parts that it was possible for them to leave traces 

 of themselves in the rocks. Hence, so far as con- 

 cerns this earliest history, we can get no record 

 of it in the rocks. 



EMBRYOLOGICAL. 



But here comes in another source of evidence 

 which helps to fill up the gap. In its develop- 

 ment every animal to-day begins as an egg. 

 This is a simple cell, and the animal goes 

 through a series of changes which eventually lead 

 to the adult. Now these changes appear for the 

 most part to be parallel to the changes through 

 which the earlier forms of life passed in their 

 development from the simple to the more compli- 

 cated forms. Where it is possible to follow the 

 history of the groups of animals from their fossil 

 remains and compare it with the history of the 

 individual animal as it progresses from the egg to 

 the adult, there is found a very decided parallel- 



