PHILOSOPHICAL ASPECTS OF EVOLUTION 



not follow, of course, that the theory of evolution does not 

 have any means at all at its disposal. What are these means 

 and what are the reasons why biologists began to think in 

 terms of evolution? 



First of all there is more or less stringent geological 

 evidence that hundred of millions years ago no life at all was 

 possible on earth. For life can only exist within very limited 

 physical conditions. Secondly, paleontological findings suggest 

 that in periods when life was possible on earth, some forms of 

 life which exist now did not exist. Thirdly, the different forms 

 of life known to us can be classified in such a way that certain 

 lines of development reveal themselves. Comparative anatomy, 

 embryology, and physiology show that the same kind of struc- 

 ture underlies forms which at first sight seem entirely different. 

 Therefore hypotheses can be proposed concerning the descent 

 of one form from the other. Sometimes it can be concluded 

 that formerly animals must have existed which are linked up 

 with animal forms living nowadays. 



Paleontological findings many times verified such theoretical 

 conclusions. These facts show that the theory of evolution is 

 not just speculation. Within certain limits, it can be verified 

 by "historical" documents. Both the general grounds and the 

 means of verifying the theory show, therefore, a more or less 

 historical character, they confirm why biology can rightly 

 be described as "natural history." To be natural history, 

 however, is only one side of biology. It also wants to be a 

 natural science. That is to say that biology wants to know the 

 general causes of evolution. Why and in what circumstances 

 did a new species develop? 



It is mainly this problem which has given rise to many 

 fervent discussions. Partly these discussions belong to the 

 realm of science. As already stated, science can directly study 

 only micro-evolution, that is to say, those differences which 

 remain within the limits of one and the same species. What- 

 ever mutations may have been discovered with the famous 

 Drosophila, the pet-animal of geneticists, these mutations do 

 not change a Drosophila into another type of animal. Con- 

 sequently, the thesis that macro-evolution is caused by the 

 same factors as micro-evolution — namely, random-mutation, 

 selection, and isolation, is for the time being an extrapolation 



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