136 THE INDUCTIONS OF BIOLOGY. 



well as from the water carrying them, and their aggregation 

 into distinct strata, is but an instance of a universal tend- 

 ency towards the union of like units and the parting of un- 

 like units {First Principles, § 163). The deposit of a crystal 

 from a solution is a differentiation of the previously mixed 

 molecules; and an integration of one class of molecules into 

 a solid body, and the other class into a liquid solvent. Is 

 not the growth of an organism an essentially similar process ? 

 Around a plant there exist certain elements like the elements 

 which form its substance; and its increase of size is effected 

 b}' continually integrating these surrounding like elements 

 with itself. Nor does the animal fundamentally differ in 

 this respect from the plant or the crystal. Its food is a 

 portion of the environing matter that contains some com- 

 pound atoms like some of the compound atoms constituting 

 its tissues; and either through simple imbibition or through 

 digestion, the animal eventually integrates with itself, units 

 like those of which it is built up, and leaves behind the 

 unlike units. To prevent misconception, it may be 



well to point out that growth, as here defined, must be dis- 

 tinguished from certain apparent and real augmentations 

 of bulk which simulate it. Thus, the long, white potato- 

 shoots thrown out in the dark, are produced at the expense 

 of the substances which the tuber contains: they illustrate 

 not the accumulation of organic matter, but simply its re- 

 composition and re-arrangement. Certain animal-embryos, 

 again, during their early stages, increase considerabty in size 

 without assimilating any solids from the environment; and 

 they do this by absorbing the surrounding water. Even in the 

 highest organisms, as in children, there appears sometimes to 

 occur a rapid gain in dimensions which does not truly 

 measure the added quantity of organic matter ; but is in part 

 due to changes analogous to those just named. Alterations 

 of this kind must not be confounded with that growth, pro- 

 perly so called, of which we have here to treat. 



The next general fact to be noted respecting organic 



