EXPERIMENTS ON LONGEVITY 



401 



mental conditions represented by the 

 standard fly husbandry of the laboratory 

 (the A of our schema) vestigial flies were 

 not able to bring their inherent vitality to 

 so complete expression in duration of life 

 as were the wild type flies under the same 

 conditions. 



This result is a specific example of the 

 general principle that the somatic expres- 

 sion of any genetic factor in any particular 

 case is in part a function of the general 

 environmental situation which exists in 

 that case. It has been demonstrated that 

 under the standard feeding conditions for 

 laboratory bred Drosophila the gene for 

 vestigial has as a part of its somatic ex- 

 pression a considerably reduced duration 

 of life as compared with the wild type. 

 There are few cleaner-cut examples of 

 Mendelian segregation to be found in the 

 whole literature of genetics than that of 

 Drosophila with respect to longevity. 

 Yet the results just described show that 

 the whole of that part of the somatic 

 expression of the vestigial gene which is 

 differential in respect of duration of life 

 disappears under another system of 

 "feeding" wild type and vestigial flies 

 (namely, complete starvation). 



An example, from quite a different field, 

 of the general principle that environmental 

 conditions must be optimal for the expres- 

 sion of the character, if reliable genetic 

 conclusions are to be drawn, is afforded by 

 the recent work of Hoffer, who showed 

 that selfed lines of maize made better 

 growth when grown in clay soils, whereas 

 hybrids of the same lines grew better in 

 loam soils. 



SEEDLING GROWTH AND LONGEVITY 



The manifestations of inherent vitality 

 can be studied in plants even more favor- 

 ably than in animals. The dry seed of a 

 dicotyledonous plant, like a canteloupe 

 (Cucumis melo~) let us say, is a complete 



but undeveloped individual. It contains 

 in the cotyledons, which are morpho- 

 logically leaves of the preformed plant 

 which the seed includes, stored nutritive 

 material sufficient to carry the seedling on 

 until the nutrition can be obtained by 

 absorption through the roots and by 

 photosynthesis. The cotyledons, and the 

 stored nutriment which they contain, are 

 an integral part, and a very important 

 part, of the total organic pattern of the 

 individual. If now we sterilize such a 

 seed, plant it on a medium which contains 

 no food material upon which the roots 

 can draw, and keep the whole preparation 

 in the dark, the growth of the etiolated 

 seedling which ensues is an expression of 

 the inherent vitality (as defined above) of 

 that individual. The seedling must draw 

 whatever nutriment it gets from endoge- 

 nous sources, which are themselves an 

 integral part of the total organized pattern 

 of the individual. Again we shall be 

 able to discuss the B element of our 

 scheme, the organization of the individual, 

 freed from the disturbing influences of A. 

 Now suppose we set up an experiment 

 of the following sort. Long glass tubes, 

 closed at one end like a test tube, are 

 made by the glass blower. In each of 

 these tubes is poured 40 cc. of a z.5 per 

 cent solution of agar in distilled water. 

 The agar has been purified by repeated 

 washing in distilled water, until all 

 soluble material has been removed from it. 

 The tubes and their contents are then 

 sterilized in an autoclave. After the agar 

 has cooled and set to a jelly there is 

 placed on its surface, with aseptic pre- 

 cautions, one canteloupe seed, which has 

 been specially prepared in the following 

 way. In the first place all the seeds for a 

 given experiment are the produce of a 

 single melon grown the year before in our 

 experimental garden. In this way ap- 

 proximate genetic homogeneity in the ex- 



