66 ORGANIC EVOLUTION — THE FACTORS 



chemical compounds, not of cells, of unicellular animals ; 

 and as the molecules of a crystal arrange themselves in 

 an aggregate which has a particular shape — the shape 

 of the crystal — so, he thinks, do the physiological units 

 of which an animal is compounded, arrange themselves 

 in an aggregate which has a particular shape — the 

 sliape of the animal. Differences in animal shapes are 

 by him accounted for by supposing that there is a 

 slight difference in their physiological units, which in 

 the aggregate produces considerable differences in 

 shape. Thus, a dog differs from a sheep because of a 

 dififei-ence in physiological units. A son resembles his 

 father because their physiological units are much alike ; 

 he differs somewhat from his father because of a slis^ht, 

 a very slight, difference in physiological units. Physio- 

 logical units according to him are modifiable by all forces 

 that impinge on the aggregate — the animal. The 

 germ, like the rest of the animal, is compounded of 

 physiological units, and, like the rest of the animal, its 

 units are modifiable by the play of forces to which the 

 aggregate is subjected. Therefore, when tlie aggregate 

 is modified by forces impinging on it, the germ is like- 

 wise modified, wherefore, when, by the assimilation 

 of food-molecules, which in it are built up into 

 physiological units similar to those composing it, growth 

 occurs, it develops or crystallizes into an organism, 

 which reproduces the modifications of the parent organ- 

 ism. In this way is the transmissibility of acquired 

 traits accounted for, and, in Mr. Spencer's opinion, to 

 doubt that transmissibility is to doubt the persistence 

 of force. 



Mr. Spencer's arguments are marshalled with great 

 skill, and the theory which he supports by them is 

 highly philosophical ; but there are fatal objections to 

 it. It is impossible to admit that that form of force 

 which causes, under fit conditions, the development of 



