1256 LIFE : OUTLINES OF GENERAL BIOLOGY 



apart from its warming influence. (3) We live in the sunshine for a 

 day or two, and we become temporarily more or less sunburnt. This 

 does not last, it has disappeared in a week or two; it is a temporary 

 adjustment of the organism to the environment. (4) We work in the 

 tropics for twenty years and are tanned. There is a positive modi- 

 fication in our skin that lasts throughout life. When we come home 

 to enjoy our leisure the tanning persists — it is a permanent modifica- 

 tion. (5) The influences of the environment may first affect the bod}" 

 and then reach the germ-cells lying within the body. That is especialh^ 



G.C 



Fig. 201 



Action and Reaction between Organism and Environment. The outer circles 

 (the Environment) inchide (A) animate, (B) Physical. (C) Chemical, and 

 (D) Mechanical influences. As the arrows (Y) show, these may penetrate 

 to different depths of the organism, to the ectodern (i), to the mesoderm 

 (2), to the endoderm (3), and even to the germ-cells (G.C). From the 

 organism there may be influences (X arrows) reaching various zones of the 

 environment. 



true of climatic influences; the body is primarily influenced, but 

 so are the germ-cells, and thus a subsequent generation. (6) The 

 environment may act as a variational stimulus. Prof. Tower shut 

 up potato beetles in a finely made cage of steel and plate glass, in 

 which by turning a handle he could alter the humidity and tempera- 

 ture. He subjected the beetles to unusual conditions of temperature 

 and humidity when the reproductive organs were at a certain stage 

 in their development. The beetles themselves are not amenable to 

 environmental influences, except of a very drastic kind, but at a 

 critical time the germ-cells seemed to be affected by the change of 

 environment, and the offspring showed many novelties, some 

 breeding true. (7) The creature may be modified by peculiarities in 



