140 THE STORY OF THE LIVING MACHINE. 



able to trace with accuracy how the somewhat 

 more generalized forms of earlier days were 

 changed to produce our modern animals. 



This fossil record has given us our best knowl- 

 edge 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 building 

 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 be- 

 ings 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 development 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 fos- 

 sil 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 

 parallelism. This parallelism between embryology 



