ANIMAL EVOLUTION ii 



The germ-cell, thrown off by the parent form, is 

 necessarily composed of the physiological units proper to 

 that parent form, and consequently possessed of certain 

 specific characters. These characters are nowhere clearly 

 defined, but it is made clear that they are not to be 

 considered as corresponding in any degree to the specific 

 structural characters of the parent or of the adult form 

 that is to be. The physiological units are conceived of 

 as having a certain 'polarity, characteristic of the species 

 to which they belong, and by reason of this polarity the 

 units react in a certain way to the incidence of external 

 forces, and in the course of cell-division and multiplication 

 are redistributed in such a way as to cause them to 

 react in a characteristic manner on one another, and thus 

 to undergo modifications, determined in part by their 

 own polarity, in part by the relations in which they 

 stand to other units, and in part to the influence of 

 external forces. These induced modifications in turn 

 induce new modifications, both in the constituent physio- 

 logical units and in the aggregate of which they form 

 a part, and the result is an increasing complexity ; but a 

 complexity of a specific kind, partly because the reactions 

 of the constituent physiological units are determined 

 by their original polarity, partly because in the normal 

 course of development of an individual animal, the 

 environment, that is to say the incident forces, are the 

 same or nearly the same, as in the development of other 

 individuals of the same species. 



On this supposition the evolution of the individual is 

 as truly a progression from the homogeneous to the 

 heterogeneous, as is the evolution of the race, and the 

 parallelism between the two is nearly complete. More- 

 over, the phenomena of inheritance and variation are 

 accounted for. For if the polarities of the units are alike, 

 and the incident forces are alike, then the reactions will 

 be alike, and the adult individual will be composed of 

 the same kind of physiological units as were contained in 



