152 THE INDUCTIONS OF BIOLOGY. 



titles. The fluids circulating throiigh the body contain 

 special units of this same order. And these diffused unita 

 are continually being deposited along with the groups of like 

 units that already exist. How purely phj'sical are the causes 

 of this selective assimilation, is, indeed, conclusively shown 

 by the fact, that abnormal constituents of the blood are 

 segregrated in the same way. Cancer-cells having begun to 

 be deposited at a particular place, continue to be deposited at 

 that place. Tubercular matter, making its appearance at 

 particular points, collects more and more round those points. 

 And similarly in numerous pustular diseases. Where 



the component units of an organ, or some of them, do not 

 exist as such in the circulating fluids, but are formed out of 

 elements or compounds that exist separately in the circulat- 

 ing fluids ; it is clear that the process of differential assimil- 

 ation is of a more complex kind. Still, however, it seems 

 not impossible that it is carried on in an analogous way. If 

 there be an aggregate of compound atoms, each of which 

 contains the constituents A, B, C ; and if round this aggre- 

 gate the constituents A and B and are diffused in un com- 

 bined states ; it may be suspected that the coercive polar force 

 of these aggregated compound atoms A, B, C, may not only 

 bi'ing into union with themselves adjacent compound atoms 

 A, B, C, but may cause the adjacent constituents A and B 

 and C to unite into such compound atoms, and then aggre- 

 gate with the mass. Should this be so, the process of differ- 

 ential assimilation, which plays so important a part in 

 organic development, will not be difiicult to understand. At 

 present, however, chemical inquiry appears to have furnished 

 no evidence either for or against such an hypothesis. 



