326 BIOLOGY 



Living organisms possess three properties, by the interaction 

 of which the present world has been formed. These are re- 

 production, heredity, and variability. That these three factors 

 are necessarily concerned is evident. Without reproduction 

 there could not have been produced the successive generations 

 which have followed each other; unless the successive genera- 

 tions had, by heredity, reproduced the characters of preceding 

 generations, there would have been no connection between one 

 type and another; and lastly, if the successive generations had 

 not shown variability, organisms would have remained in a sta- 

 tionary condition, without any opportunity for change. That 

 these forces have been sufficient to account for the develop- 

 ment of the organisms inhabiting the world, i. e., to explain 

 the origin of the living machines, is not so evident. To show 

 how the result has been brought about has been the endeavor 

 of biological discussion for the last half-century. The property 

 of reproduction we have already considered. The considera- 

 tion of heredity and variation remains. 



Heredity. The general rule in reproduction is that the off- 

 spring grow into individuals like their parents, the repetition 

 of the parent being spoken of under the name of heredity. 

 Heredity must not be looked upon as any special force or law, 

 but merely as a word expressive of the fact that one generation 

 repeats itself in the next. It is evident that this process of 

 repetition cannot be exact, since most animals have not one 

 but two parents, and an individual that has a father and a 

 mother cannot be exactly like both of them if they are in the 

 slightest degree unlike. Since no two animals are exactly alike, 

 the natural conclusion would be that the offspring would be a 

 compromise between its two parents. Successive generations 

 are thus not identical, but constantly show differences from 

 their parents. Heredity means, then, that successive genera- 

 tions resemble their parents as closely as is compatible with 

 the fact that the individual has two parents, and cannot be 

 like both. 



