CHAPTER XII 



No sketch of the principles of Biology, even though 

 as brief as the present one, would be complete 

 without a reference, however short, to the subject 

 which forms the title of this chapter. 



Every day experience teaches us that both in 

 the plant world and in the animal world there are 

 many different types of organism, and that these 

 may be arranged in a gradually ascending series from 

 the most lowly unicellular types to the highest and 

 most complicated forms, culminating in a daisy or a 

 tree, on the one hand, and in man himself on the 

 other. Under each type there are endless sub- 

 types, and under these, again, yet other subordinate 

 types. It becomes at once evident that some expla- 

 nation of the relationships of these types to each 

 other must be forthcoming if we are to believe in 

 life on the globe as an organic unity. Round this 

 question there has for long raged a vigorous con- 

 troversy, some authorities holding that each type 

 represented a distinct act of creation, others holding 

 that types were not immutable, but that there existed 

 a family relationship between them, if one had only 

 the data necessary to construct a genealogical tree. 

 Without entering in any detail into this controversy 

 let us attempt to gain some insight into the funda- 

 mental premises which must underlie any theoretical 

 explanations that may be advanced. 



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