THE GENERAL CONDITIONS OF LIFE 313 



tology reconstructs to a certain degree the organic world that 

 inhabited the earth's surface at the times when the strata 

 were formed. Thus the ancestry of existing animals and plants 

 is learned. It is seen that existing forms are very similar to 

 those that occur in the latest strata ; that the forms become 

 more dissimilar the farther we go toward the earliest strata ; and 

 that large groups of organisms, which are now considered to be 

 widely separated from one another, have in the older strata 

 common ancestors, which combine in themselves the characteristics 

 of several groups. In the very earliest strata are found lower 

 animals and plants only, no vertebrates and no flowering-plants 

 occur. For every one who is not wedded to a blind, supernatural 

 faith concerning creation, and who does not prefer, in accordance 

 with the biblical account, to think of every form of organism as 

 proceeding by itself from the hand of a personal Creator, there is only 

 .a single natural explanation of all palseontological facts ; namely, 

 that the whole world of organisms, living to-day and living in the 

 past, forms a single, great genealogical tree, the germ of which was 

 the first living substance that appeared upon the earth. This 

 germ developed into a mighty growth with innumerable branches 

 .and twigs and leaves ; its last shoots are seen in the organisms 

 of to-day, its older branches lie buried in the earth. Unfortunately, 

 the pala3ontological record is very imperfect ; for, on the one hand, 

 only a very small fraction of the earth's strata is accessible to 

 investigation the greater portion of the crust is covered by the 

 .sea ; and, on the other hand, the preservation of organisms is very 

 incomplete, since they can be imbedded only under very definite 

 conditions without becoming destroyed by the impact of the 

 waves, by decomposition, etc. In fact, organisms that did not 

 possess protecting skeletal parts have been preserved hardly 

 at all, because their delicate bodies disintegrated immediately 

 after death. It thus comes about that in the investigation 

 of the oldest and simplest organisms, which possessed no 

 protecting skeletal parts, the palseontological record fails. 



Comparative anatomy deals with the second record, which is 

 presented in the homologies of the individual organs of existing 

 organisms. By the dissection of organisms into their smallest 

 parts and by the comparison of individual organs and systems of 

 organs belonging to different groups of organisms, comparative 

 anatomy establishes the fact that as regards their essential 

 organic systems certain groups of organisms agree with others 

 to a certain extent. This fact can be interpreted rationally only 

 by the assumption of a natural relationship between such 

 organisms ; in general such a relationship is closer, the more 

 homologies occur, and the more remote, the more differences are 

 present ; for the homologies can be due only to the fact that 

 iit some time in the early past the organisms had common 



