130 A TEXT-BOOK OF BOTANY. 



The theory of evolution has as its basis the idea that the 

 existing species of plants and animals are the descendants of earlier 

 forms. It holds that there is an unbroken line of descent from 

 the beginning of life on the earth, but that during the long ages 

 the successive descendants gradually changed in appearance from 

 their ancestors until we find the forms of the present day. 



Nearly all branches of biological science give evidence in 

 support of the theory of evolution. Embryology, for instance, has 

 shown that in its development the individual during its life, begin- 

 ning with the fertilization of the egg-cell, passes through a series 

 of stages which are thought to represent the same series of stages 

 through which the whole race before it passed. The develop- 

 ment of the individual (i.e., ontogeny) represents in a very brief 

 space of time the evolution of the race (i.e., phylogeny). In 

 other words, " ontogeny epitomizes phylogeny." 



Another branch of science which is bringing forth new evi- 

 dence is the branch called paleontology. This subject has to do 

 with the study of fossil remains and with the time they existed 

 on the earth in the living state. It has been found that fossils from 

 the dififerent series of formations that make up the earth's outer 

 crust represent a regular advancement from the very simplest types 

 to those which are most complicated, right up to the most recent 

 forms. In not a single instance has a highly, developed form been 

 found in a layer of rocks representing an early stage in the earth's 

 history. 



Every scientist of the present time, probably without exception, 

 believes in the theory of evolution, but there is a great diversity of 

 opinion as to how it should be explained. This diversity of 

 thought, instead of disproving the idea of evolution, is making its 

 truth more generally felt. The problem, then, which is confront- 

 ing the scientist is not to prove that evolution is a truth, but to 

 explain it; to show how new forms may arise from old ones, — 

 that is, to account for the origin of species. Among the many 

 explanations the following have become most conspicuous: 



Environment. — It was naturally thought at first that the 

 natural conditions under which organic life developed must have 

 a certain effect upon the individual, thereby bringing about a cer- 

 tain modification which would be transmitted in successively 



