INTRODUCTION 7 



the individual body is organised to act as a single mechanism, 

 that the same degree of organisation obtains in the individuals 

 of all phyla. The more correct reasoning, it seems to me, is to 

 start from the idea of units of protoplasm growing wherever 

 food was available, and to recognise that there are definite 

 ascertainable principles of growth, such as, that stimulation 

 of certain kinds by external forces causes growth to take 

 certain directions, and that new growth has a tendency, 

 inherent in the processes of life, to resemble old growths in 

 form. Similarity of parts is thus but a phenomenon of the 

 same kind as heredity, and the characteristic forms of the 

 various sub-kingdoms are not the most useful selected out of 

 a number that occurred, but are the definite and only possible 

 results of the expansion of growing organisms under different 

 conditions. The laws of growth and the effects of con- 

 ditions are not fully known, but it is possible to investigate 

 them. 



We will next briefly consider the extent of adaptation or 

 utility in the peculiarities which distinguish species. The 

 conception of species has played a very important part in 

 the history of zoological science. Darwin, as we know, called 

 his theory a theory of the origin of species. If distinct 

 species of animals were not separately created, how did they 

 arise ? Darwin's answer was, by natural selection or the 

 survival of the fittest. In artificial breeding man preserves 

 and keeps separate those individuals which have certain char- 

 acters either useful or agreeable to him, and so forms numerous 

 breeds out of one stock, or modifies one stock to a great 

 degree from its original condition. In nature the struggle 

 for existence and the survival of the fittest are alleged to 

 produce similar results. One set of individuals survives by 

 reason of one slight advantage, another of another. Accord- 

 ing to this view the primary and essential differences between 

 species must be differences of adaptation. 



