chap, i.i HEREDITY AND VARIABILITY. 15 



the third and fourth metacarpals one with the other 

 (law 3) ; the result of this last process being the 

 formation of a bone which at its lower end only 

 gives any obvious indication of its primitively double 

 nature. 



The characteristics of protoplasm which appear to 

 be the determining factors of evolution, are (1) its 

 power of producing an organism like to itself; and (2) 

 the fact that no child or parent, or any two children, 

 are exactly similar one to another. The first of these 

 principles is known as that of heredity, the second 

 as that of variability. It is obvious that the 

 second principle only comes into action because of the 

 differences in the surroundings of every individual 

 plastid ; the greater the homogeneity of the surround- 

 ings, the greater the likenesses between the plastids. 

 The law of heredity may consequently be compared to 

 the first law of motion (Gasquet). 



Organisms, therefore, tend to resemble their 

 parents, but, being more or less differently affected by 

 surrounding media and objects, diverge more or less 

 from the parent stock ; the greater the differences 

 in environment, the greater the differences between 

 parent and child. This is a fact so well known to 

 us all that we need not enlarge upon it here. 



ANIMALS AND PLANTS. 



While the conviction that there is an essential 

 unity between animals and plants may be taken as 

 one of the most important results of modern biology, 

 we have to note that along the two lines of organisa- 

 tion the constituent protoplasm has, on the whole, de- 

 veloped special characteristics. In other words, we are 

 not able always to say definitely whether a given uni- 

 cellular- organism is an animal or a plant ; but we can 

 always with certainty point to the differences which 

 distinguish a rose from a bee. 



