60 Heredity and Environment 



tions represented by temperature, pressure, moisture or other 

 extrinsic factors. 



4. Heredity and Environment. Unquestionably the factors 

 or causes of development are to be found not merely in the germ 

 but also in the environment, not only in intrinsic but also in 

 extrinsic forces; but it is equally certain that the directing and 

 guiding factors of development are in the main intrinsic, and 

 are present in the organization of the germ cells, while the en- 

 vironmental factors exercise chiefly a stimulating, inhibiting or 

 modifying influence on development. In the same dish and un- 

 der similar environmental conditions, one egg will develop into a 

 worm, another into a sea urchin, another into a fish, and it is 

 certain that the different fate of each egg is determined by con- 

 ditions intrinsic in the egg itself, rather than by environmental 

 conditions. We should look upon the germ as a living thing, and 

 upon development as one of its functions. Just as the character 

 of any function is determined by the organism, though it may be 

 modified by environment, so the character of development is 

 determined by heredity, i.e., by the organization of the germ 

 cells, though the course and results of development may be modi- 

 fied by environmental conditions. 



SUMMARY 



In conclusion, we have briefly reviewed in this chapter the well 

 known fact that every living thing in the world has come into 

 existence by a process of development ; that the entire human per- 

 sonality, mind as well as body, has thus arisen; and that the 

 factors of development may be classified as intrinsic in the organi- 

 zation of the germ cells, and extrinsic as represented in environ- 

 mental forces and conditions. The intrinsic factors are those 

 which are commonly called heredity, and they direct and guide 

 development in the main ; the extrinsic or environmental factors 

 furnish the conditions in which development takes place and 

 they modify, more or less, its course. 



