CHAPTER XII 



THEORETICAL BIOLOGY 



In the preceding chapters, in which we have considered the 

 more elementary facts of biology in general and of zoology in 

 particular, we have from time to time noted matters of consider- 

 able theoretical interest. Many volumes have been written on 

 these subjects, which constitute the philosophy of biology, and 

 there has been as much diversity of opinion as there is on most 

 philosophical subjects. It will be interesting to follow brieflv 

 the more important lines of reasoning which have been suggested 

 by various thinkers from the time when the subject was first 

 scientifically considered, at about the beginning of the nineteenth 

 century, down to the present day. 



As soon as scientists were sufficiently familiar with the struc- 

 ture of animals and plants to recognize that the species could 

 be grouped into genera, the genera into families and so on, the 

 questions arose: How did all the various organisms come into 

 existence ? Have there always been as many species on the earth 

 as there are to-day, and will they remain the same for all time 

 to come? Linnaeus replied: There are as many species as an 

 Infinite Being created in the beginning; or, to put it in other 

 words, each species living to-day came into existence by a special 

 act of creation when life first appeared on this earth. But when 

 fossil remains were more carefully studied and as they became 

 more and more numerous, it was soon apparent that in past 

 ages the earth had very different inhabitants from the present. 

 Scientists who held to the theory enunciated by Linnaeus at- 

 tempted to explain this fact bv assuming that each geological 

 period was, so to speak, complete in itself, and was suddenly 

 brought to a close by some mighty convulsion or revolution of 

 the forces of nature, a cataclysm, as it was called, which exter- 



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