28 



CAUSES OF VARIATION 



As Morgan remarks, 1 " The growth of the new part at the 

 expense of the old tissues is a phenomenon of the greatest im- 

 portance, an explanation of which will involve, I think, the most 

 fundamental questions pertaining to growth." To this remark it 

 may be added that the phenomenon is also of the greatest impor- 

 tance in its bearing upon the relative stability of living matter. 

 In so far as protoplasm is worked over to serve new purposes, it 

 shows wonderful elasticity ; but the fact that 

 the organism preserves or completes its plan 

 at almost any expense, even to itself, betrays 

 a wonderful persistence on the part of the 

 original plan. 



It is known that the starving cat or dog 

 will replace a large share of the dry matter 

 of the body with water, and sacrifice all other 

 activities to the vital processes. Plants grow- 

 ing with no nitrogen supply save what is 

 contained in the seeds will soon reach the 

 maximum of possible development, but will 

 continue to produce new leaves at the ex- 

 pense of the old ones, 2 as the rapidly grow- 

 ing stalk of the century plant feeds on its 

 thickened leaves, or the beet and carrot feed 

 on their thickened roots. 



Effects of light upon regeneration. 3 In the 



same individual after cage Q f p l ants on l y 4 ^ O eS light Seem to affect 

 being kept without food 



for four months and regeneration, and iri them the influence 

 appears to be confined to the blue rays. 

 " Herbst observed that when the eye of cer- 

 tain Crustacea is cut off, sometimes an eye and sometimes an 

 antenna is regenerated." Experiments were conducted to see 

 whether light might be the factor which determined whether the 



1 Morgan, Regeneration, pp. 27-29. 



2 The writer saw the original clover plants in the first Rothamsted series for test- 

 ing the nitrogen-gathering powers of root tubercles. One of these plants had no 

 source of nitrogen but the seed. It was two years old, still producing new leaves 

 as the old ones died down, but it had never blossomed, nor was it able to pro- 

 duce more than four leaves at any time. 



3 Morgan. Regeneration, pp. 29-30. 4 Ibid. p. 78. 



FIG. 40. Effect of star- 

 vation upon the pla- 

 in arian 



A, well-fed worm ; ff, the 



thirteen days. After 

 Morgan 



