80 BIOLOGY 



the race was not sufficiently plastic to save themselves, 

 and this latter reason may explain why we have no 

 longer giant reptiles and amphibians ; or perhaps the 

 evolution-momentum of some particular feature got, as 

 it were, out of hand, and produced a character which was 

 harmful instead of beneficial to the race. One thing is 

 certain, that the exact reasons for the extinction of many 

 forms will never be known. 



We have given here a very brief sketch of the geo- 

 logical record, but still sufficient to show that, in spite 

 of all its imperfections, and of all its puzzles, it must 

 not be neglected by the student who would form an 

 intelligent opinion of the history of living things. 



CHAPTER XV 



HEREDITY 



WHAT is heredity ? It is the name given to the pro- 

 perty that is passed on from one generation to the next, 

 a property which ensures, on the whole, the transmission 

 of the characteristic features of an organism to its off- 

 spring. The present generation of any particular race 

 is related to the antecedent one in virtue of this 

 property, and will be related to the generation it 

 begets in a similar manner ; hence heredity may be 

 defined as the organic relation between successive 

 generations. 



In the majority of cases, organisms develop from an 



