4 COMPARATIVE ANATOMY 



During infancy and youth the gain by food is greater than the 

 loss, and so the animal grows ; during maturity the income 

 and expenditure are practically balanced. In old age the 

 expenditure is in excess of the income, so that decay sets in 

 and the organism finally perishes. 



It perishes, but not entirely, for it has the capacity, which 

 is another great characteristic of living things, of reproducing 

 its like. On reaching maturity a part of it, generally an 

 infinitesimal part, is separated : this part lives, absorbs 

 nourishment and grows into a new organism which resembles 

 in every essential particular the parent form from which it was 

 separated. It is in virtue of the resemblance of the offspring 

 to the parent, commonly called heredity, that species retain 

 that fixity of form and structure to which we have alluded. 



These facts are sufficiently familiar to all, but the con- 

 sequences of them are very seldom understood. The fact 

 that food is necessary, both to animals and plants, makes 

 them dependent on those external physical conditions which 

 are generally summed up in the term environment. Plants 

 are as much, and even more dependent on their environment 

 than animals, but their relations to it are quite different. 



A plant is able to obtain its food in a manner which is quite 

 impossible to animals.* The substance of which all living 

 things, both plants and animals, are composed is known as 

 protoplasm. We shall study it more closely hereafter, and 

 need only state here that it is an albuminous body of great 

 chemical complexity, composed of the elements Carbon, 

 Hydrogen, Oxygen, Nitrogen, Sulphur, Phosphorus. It is 

 obvious that an organism, in order to repair the material of its 

 body must be supplied with all these elements. Plants 

 are able, in virtue of the property possessed by their green 

 colouring matter, chlorophyll, to obtain their carbon from 

 the minute quantities of carbon dioxide present in atmospheric 

 air. So part of their food is gaseous. The other constituents 

 they obtain in solution in water, by means of their roots, so that 

 the remainder of their food is absorbed in a liquid form. If 

 a plant is supplied with a weak solution of certain mineral 

 substances, and is kept in the air and in sunlight (for 

 chlorophyll is only active in sunlight), it is able to construct 

 protoplasm out of these simple mineral and gaseous com- 

 * With the few exceptions to be mentioned later. 



