INTRODUCTION 



organisms must result which are, in the highest degree, fitted and 

 adapted to their environment. Thus, by the survival of the fittest, 

 through natural selection, that adaptability to the environment is 

 gradually evolved which is such a striking characteristic of organic 

 life. That the transitional forms in this process of phylogenetic de- 

 velopment no longer exist, is accounted for in the theory of natural 

 selection by the assumption that the struggle for existence must 

 necessarily have been most severe between similar organisms. For 

 similar organisms must have similar necessities, and the new and 

 better-equipped forms must ultimately prevail over the original less 

 specialised organisms, which, thus deprived of the essential requisites 

 for their existence, finally disappear. 



Although the great importance of natural selection in the develop- 

 ment of the organic world has been fully recognised by most 

 naturalists, the objection has been raised that it alone is not a 

 sufficient explanation of all the different processes in the phylogeny 

 of an organism. Attention has been called to such organs as would 

 be incapable of exercising their function until in an advanced 

 stage of development, and so could not originally have been of any 

 advantage in a struggle for existence. How could natural selection 

 tend to develop an organ which would be useless so long as it was 

 still in a rudimentary condition ? This objection has led to the 

 supposition of an internal force residing in the substance of the 

 organisms themselves, and controlling their continuous development 

 in certain definite directions. Many naturalists, indeed, have gone so 

 far as to affirm that only less advantageous qualities have been 

 affected by the struggle for existence, while the more advantageous 

 have been uninfluenced by it. 



The phylogenetic changes in the species have been so gradually 

 accomplished as to have escaped observation, and indirect evidence of 

 their existence is all that can be obtained. 



If the higher organisms have been evolved from the lower, there 

 must, at one time, have been no sharp distinction between plants and 

 animals. The simplest organisms which now exist are in all proba- 

 bility similar to those which formed the starting-point in the phylo- 

 genetic development of animal and vegetable life ; and it is still 

 impossible to draw a sharp distinction between the lower forms of 

 plants and animals. The walls which surround the elementary organs 

 of the plant body, and the green colouring matter formed within them, 

 have been cited as decisive indications of the vegetable character of an 

 organism. Surrounded by firm walls, the living substance becomes 

 more isolated, and, consequently, independence of action in plants, as 

 compared with animals, is diminished. By means of the green colour- 

 ing matter, plants have the power of producing their own nutritive 

 substances from certain constituents of the air and water, and from the 

 salts contained in the soil, and are thus able to exist independently ; 



