282 Biology in America 



ment of the river to its channel, or the snow drift to the 

 wind. 



Yet another manifestation of life is its irritability or power 

 of response to stimuli. Examples of this are so common that 

 it is merely trite to repeat them. There is no form of life 

 so primitive or so sluggish as to escape this universal law. 

 But is tliis phenomenon limited to life alone? Does not life- 

 less matter also respond to stimuli, or changes in its environ- 

 ment? Examples of such changes must occur to the mind 

 of everyone — changes in volume or in state, whether solid, 

 liquid or gaseous, in response to changes in temperature or 

 pressure, are among the most familiar instances of these 

 responses. If a metal be heated its electrical conductivity 

 is decreased, sound travels faster the higher the temperature, 

 while atmospheric conditions will materially affect the 

 messages flashed from the wires of the radio. While the 

 responses of living things and changes in their environment 

 are infinitely more complex and indirect than are those of 

 the non-living, yet the same principle holds true for both, 

 and when we know more of the mechanism of life it may be 

 possible to resolve its complex reactions into their simpler 

 terms. 



Yet one great characteristic of life remains, namely, repro- 

 duction. The development of a human being with his myriad 

 cells, more varied in form than the manifold parts of the 

 most complicated machine, ranging in size from the tiny 

 corpuscles of the blood, less than one four-thousandth of an 

 inch in size, to the motor nerve cells of the spinal cord, which 

 may reach a length of over three feet; and including the 

 intricate structures of the brain by which are performed all 

 the wonderfully complex functions of the human body, in- 

 cluding the as yet inscrutable processes of thought ; all these 

 coming from an apparently simple cell a little more than one 

 one-hundredth of an inch in size, is a wonder beside which the 

 magic of an Aladdin or the miracles of holy writ fade into 

 ghostly paleness. The enthusiast in the ranks of the mech- 

 anists has attempted however to remove even this most 

 distinctive feature of living things, by showing that non- 

 living matter may in a certain sense reproduce itself, as new 

 crystals form in an evaporating salt solution. However feeble 

 such a comparison may be, it is nevertheless true that all 

 phases of reproduction — the growth of the germ cells, their 

 union, the entrance of the spermatozoon, the division of the 

 fertilized egg, the growth and differentiation of the tissues 

 are all intimately associated with physico-chemical changes 

 taking place in these cells, and can, as we shall see later, 

 to a certain extent at least be induced by artificial means. 



