3i6 



READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



dates from June 3 to January 3. By raising these animals under 

 constant temperature conditions and varying the strength of the 

 nutrient solution, Woltereck proved that the relative size of body 

 parts varied with the food. In Fig. 54 the percentages of head height 

 to shell in length are plotted as abscissas and the numbers of indi- 

 viduals as ordinates. Animals from three strengths of nutrient media 

 were measured, the curves of those from the weaker, the medium and 

 the richer media being shown at mi, ma and m3 respectively. 



d) Moisture and plumage color. Beebe experimented with the 

 pigeon, Scardafella inca. This species, as found in North and Central 

 America, is very constant in color of plumage, but in the moist tropics 



50 55 CO 65 70 ' 75 80 



m2 



85 



90 95 



ni3 



Fig. 54. — Schematic curves of head height in Hyalodaphnia as grown in 

 media of three different food values. {From Bahcock arid Clausen, after 

 Woltereck.) 



the following darker colored forms occur: in Honduras, dialeucos; in 

 Venezuela, ridgwayi; in Brazil, hrazilieiisls; and these differ in the 

 amount of pigment in the feathers. By subjecting the birds of the 

 northern type to an especially moist atmosphere, Beebe caused them 

 to be so influenced that with each new moulting, whether natural or 

 artificially induced, they always developed darker feathers. Thus a 

 wild bird having pigment in 25.9 per cent of its area, would have after 

 the second moulting under experimental conditions, 38 per cent and 

 after the third, 41.6 per cent. Thus during the experiment the 

 typical form assumed the appearance of the three other forms and 

 finally developed plumage markings which have never been seen in 

 nature. Fig. 55 shows the type form, inca, the three geographical 

 variants, and the darkest artificially produced form. 



