PROOFS OF OEGAKISIKG MIIS'J) 315 



the size and shape characteristic of the species, Avhile each of 

 the often very diverse feathers grows in its right place, and re- 

 produces the various tints and colours on certain parts of every 

 feather which go to make up the characteristic colours, markings, 

 or ornamental plumes of each species of bird, presents us with 

 the most remarkable cases of heredity, and of ever-present ac- 

 curately directed growth-power, to be found in the whole range 

 of organic nature. 



The Nature of Growth 



The growth of every species of organism into a highly com- 

 plex form, closely resembling one or other of its parents, is so 

 universal a fact that, \vith most people, it ceases to excite won- 

 der or curiosity. Yet it is to this day absolutely inexplicable. 

 No doubt an immense deal has been discovered of the mech- 

 anism of growth, but of the nature of the forces at work, or of 

 the directive agencies that guide and regulate the forces, we 

 have nothing but the vaguest hints and conjectures. All 

 growth, animal or vegetable, has been long since ascertained to 

 begin with the formation and division of cells. A cell is a 

 minute mass of protoplasm, a substance held to be the physical 

 basis of life. This is, chemically, the most complex substance 

 known, for while it consists mainly of four elements — car- 

 bon, hydrogen, nitrogen, and oxygen — it is now ascertained 

 that eight other elements are always present in cells composed 

 of it — sulphur, phosphorus, chlorine, potassium, sodium, 

 magnesium, calcium, and iron. Besides these, six others are 

 occasionally found, but are not essential constituents of pro- 

 toplasm. These are silicon, fluorine, bromine, iodine, alumi- 

 nium, and manganese.^ 



Protoplasm is so complex a substance, not only in the num- 

 ber of the elements it contains, but also in the mode of their 

 chemical combination, that it is quite beyond the reach of 

 chemical analysis. It has been divided into throe groups of 

 chemical substances — proteids, carbohydrates, and fats. The 



1 Verworu's General Physiology, p. 100. 



