108 THE TNDtJCTIONS OF BIOLOGY. 



on observing that they both result in the same waJ^ ITie 

 segregation of different kinds of detritus from each other, aa 

 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, % 123). The deposit of a crystal 

 from a solution, is a differentiation of the previously mixed 

 atoms ; and an integration of one class of atoms into a solid 

 body, and the other class into a liquid solvent. le not the 

 growth of an organism a substantially similar process ? 

 Around a plant there exist certain elements that are like 

 the elements which form its substance ; and its increase of 

 eize is effected by continually integrating these surrounding 

 like elements wdth itself. Nor does the animal fundament- 

 ally differ in this respect from the plant or the crystal. Its 

 food is a portion of the environing matter, that contains some 

 compound atoms like some of the compound atoms constitut- 

 ing its , tissues ; and either through simple imbibition or 

 through digestion, the animal eventually integrates with it- 

 self, 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 

 distinguished 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 illustrato 

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

 arrangement. Certain animal-embryos, again, during their 

 early stages, increase considerably in size without assimil- 

 ating 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, that 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 



