SPECIES IN THE MAKING in 



permanence of essential conditions, a definite stability 

 and limitation to its change or variation in response to 

 variations of its environment. That part of the living 

 substance which in all but the lowest plants and animals 

 is set aside during growth to form the eggs and sperms 

 by which they multiply or " reproduce " themselves, is 

 called the " germ-plasm," and is peculiarly sensitive to 

 variations in (that is a change in) the environment of the 

 plant or animal. 



New conditions of life (locality and climate) — unusual 

 food or reproductive activity — act often in a powerful way 

 upon the germ-plasm and cause it to vary — that is to say, 

 they alter some of its qualities, though not necessarily dis- 

 turbing in any way the general living substance of the 

 organism so far as to produce any important change per- 

 ceptible to the human eye. In consequence, the young pro- 

 duced after such disturbance of the germ-plasm are found 

 to differ more from their parents than in cases where no 

 such disturbance has been set up by the natural never- 

 ceasing variation of the surrounding world. This fact is 

 well known to horticulturists and breeders, and is made 

 use of by them. When a gardener wishes to obtain vari- 

 ations of a plant from which to select and establish a new 

 breed, he deliberately sets to work to disturb — to shake 

 up, to act upon in a tentative, experimental way — the 

 germ-plasm of one or more parent plants by change of 

 soil, climate, food and often by cross-fertilizing them with 

 another breed or variety. In this way he to some extent 

 " breaks " the constitutional stability of the germ-plasm of 

 the plant and obtains abundant " variations " in the off- 

 spring. These are not precisely foreseen, and show 

 themselves in all parts of the new generation. But some 

 of them are what the gardener wants, and are " selected " 

 by him for retention, rearing and breeding. 



The response of the germ-plasm of organisms to the 



