This fossil record has given us our best knowledge of the course by which the present living world has been brought into its existing condition.  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 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 concerns 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 complicated 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 parallelism.  This parallelism between embryology and past history has been of great service in helping us toward the history of the past.  At one time it was believed that it was the key which would unlock all doors, and for a decade biologists eagerly pursued embryology with the expectation that it would solve all problems in connection with the history of animals.  The result has been somewhat disappointing.  Embryology has, it is true, been of the utmost service in showing relationships of forms to each other, and in thus revealing past history.  But while this record is a valuable one, it is a record which has unfortunately been subject to such modifying conditions that in many cases its original meaning has been entirely obliterated and it has become worthless as a historical record.  These imperfections in regard to the record were early seen after the attention of biologists was seriously turned to the study of embryology, but it was expected that it would be possible to correct them and discover the true meaning underlying the more apparent one.  Indeed, in many cases this has been found possible.  But many of the modifications are so profound as to render it impossible to untangle them and discover the true meaning.  As a result the biologist to-day is showing less confidence in embryology, and is turning his attention in different directions as more promising of results in the line desired.