610 



CONTINUITY OF LIFE 



Fig. 24-17. The individual is a result of two forces, one due to the inherited genes and the other to the surrounding 

 environment which begins with the zygote and continues throughout the life of the individual. These various steps 

 as sketched here are explained in the text. 



to give the genes in each person ample op- 

 portunity to express themselves to their 

 greatest capacity. Therefore, while good 

 genes are essential for good stock, they are 

 not enough. Good genes in a good environ- 

 ment is the ideal goal. 



There is experimental evidence to dem- 

 onstrate further the intricate relationship 

 between environment and heredity. In 

 studying the life span in fruit flies, Ray- 

 mond Pearl found that the vestigial-winged 

 forms lived only half as long as the wild 

 type in a closed milk bottle. If he increased 

 the number of flies of the wfld type he cut 

 their lifetime to that of the vestigial-wing-ed 

 flies. If both types were starved they lived 

 about the same length of time. Certainly 

 the environment limited the capacity of the 

 genes. Similarly in the Himalayan rabbits 

 previously discussed (p. 596), the chilled 



regions of the body grew black fur while 

 all other parts grew white. If the rabbit is 

 chilled in a refrigerator, for example, it will 

 develop black hair all over; conversely, if 

 it is warmed up to relatively high tempera- 

 tures it will develop white, tips and all. In 

 this case, body temperature limits the ex- 

 pression of the genes. It is obvious in all 

 these instances that the genes lay the pat- 

 tern for capacity whereas the environment 

 determines whether or not that full capac- 

 ity will be realized. 



The close interaction of these two forces, 

 environment and heredity, can be still fur- 

 ther emphasized by following through the 

 development of an organism, say a human 

 being, from fertilized egg to young adult 

 (Fig. 24-17). Begining with the fertilized 

 ovum, we have a cell consisting of cyto- 

 plasm, controlled by the genes (G) and 



