INTRODUCTION. iii 



ley tells us : " In the spring, a wheat-field is covered with small green plants. These 

 grow taller and taller until they attain many times the size which they had when they 

 first appeared ; and they produce the heads of flowers which eventually change into 

 cars of corn. 



" In so far as tliis is a jarocess of growth, accompanied by the assumption of a defi- 

 nite form, it niiglit be eomj^ared with the growth of a crystal of salt in brine ; but, on 

 closer examination, it turns out to be something very different. For the crystal of salt 

 grows by taking to itself the salt contained in the brine, which is added to its exterior ; 

 whereas the plant grows by addition to its interior ; and there is not a trace of the 

 characteristic compounds of the plant's body, albumen, gluten, starch, or cellulose, or 

 fat, in the soil, or in the water, or in the air. 



" Yet the plant creates nothing, and therefore the matter of the proteids and amy- 

 loids and fats which it contains must be supplied to it, and simply manufactured, or 

 combined in new fashions, in the body of the plant. 



" It is easy to see, in a general way, what the raw materials are which the plant 

 works up, for the plant gets nothing but the materials supplied to it by the atmosphere 

 and by the soil. The atmosjjhere contains oxygen and nitrogen, a little carbonic acid 

 gas, a minute quantity of ammoniacal salts, and a variable proportion of water. The 

 soil contains clay and sand (silica), lime, iron, potash, phosphorus, sulphur, ammoniacal 

 salts, and other matters which are of no imiwrtance. Thus, between them, the soil 

 and the atmosjihere contain all the elementary bodies which we find in the plant ; but 

 the plant has to separate them and join them together afresh. 



"Moreover the new matter by the addition of which the plant grows is not applied 

 to its outer surface, but is manufactured in its interior; and the new molecules are 

 diffused among the old ones." 



Living beings also reproduce their kind. The corn bears seed, the hen lays eggs. 

 Minerals cannot reproduce, so that we see that organic beings differ from minerals in 

 three essential characteristics : they contain and are built up from protojilasm ; they 

 grow from within, and they reproduce by seeds, germs or eggs. 



As Huxley again says : " Thus there is a very broad distinction between mineral 

 matter and living matter. The elements of living matter are identical with those of 

 mineral bodies ; and the fundamental laws of matter and motion apply as much to 

 living matter as to mineral matter; but every living body is, as it were, a com- 

 plicated piece of mechanism, which 'goes' or lives, only under certain conditions. 

 The germ contained in the fowl's egg requires nothing but a supply of warjiitli within 

 certain narrow limits of temperature, to build the molecules of the egg into the body of 

 the chick. And the process of development of the egg, like that of the seed, is 

 neither more or less jnysterious than that in virtue of which the molecules of water, 

 when it is cooled down to the freezing point, build themselves up into regular crystals." 



The Dipferences between Plaxts and Animals. 



We now come to the differences between plants and animals ; and here the dis- 

 tinctions are more or less arbitrary. As we have remarked elsewhere, — 



"It is difficult to define what an animal is as distinguished from a plant, when we 

 consider the simplest forms of either kingdom, for it is impossible to draw hard and 

 fast lines in nature. In defining the limits between the animal and vegetable king- 

 doms, our ordinary concejition of what a plant or animal is Avill be of little use in 

 dealing with the lowest forms of either kingdom. A horse, fish, or worm differs from 



