DIMINISHING ENVIRONMENTAL INFLUENCE 317 



in the amount or quality of heat, light, food, gravity, soil, manure and 

 atmosphere may be easily verified. 



Low Metazoa 



Among the low metazoa the influence of temperature, light and 

 food is not so striking as among plants, still the changes may be 

 called remarkable. For instance, the body-length of Echinus larvas 

 may be made, by raising the temperature, to increase about 25 per 

 cent, of the average growth attained under a low temperature, while 

 the arm-lengths in certain forms are at the same time increased from 

 200 to 300 per cent. If the number of larva? of Echinus and Stron- 

 gylocentrotus, which are growing together, be increased from under 

 1,500 per liter to over 3,000 per liter the mean length of the anal 

 arms and the oral anus may be made to diminish from the figures 

 121.2 and 118.4 to 56.6 and 68.5, respectively. 7 No modifications like 

 this have been induced among the higher vertebrates. 



In the embryology of placental mammals the dorsal or ventral sur- 

 face may be facing either up or down, right or left ; all forces work out 

 their destiny in disregard to the force of gravity. This force of gravity 

 so important in moulding plants has been proved to be also very influ- 

 ential in the development of hydroids. Pieces of Antennularia an- 

 tennia produce new stems that grow upward, and stolons that turn 

 downward. Even if the piece is inverted the root arises from the lower 

 end and the stem from the upper. Driesch observed in a species of 

 Seriularia that whenever he altered the position of the piece the new 

 growth changed its position so that the new part turned away from 

 the center of the earth. 8 



The most remarkable of all experiments on low metazoa are the 

 regeneration experiments. Since the exposed cells are subjected to a 

 very considerable alteration of their normal environment, regeneration 

 experiments may be considered as coming under the head of modifica- 

 tion experiments. The power of regeneration possessed by worms and 

 hydroids is so well known that only a passing reference is needed here. 

 It is significant that among worms the tail will regenerate more than 

 the head and that the reproductive organs, if removed, never regenerate 

 at all, and the worm remains " incapable of reproducing itself." 9 

 This last fact is interesting as supporting the view that germ-cells 

 are never reproduced from somatic cells of any kind, much less from 



6 Conf. Morgan, "Exp. Zool.," pp. 44, 265, 266. Vernon, "Variation," pp. 

 228, 245, 249, 262, 269, 282, 284-286, 312-314. C. B. Davenport, "Exp. Mor- 

 phology," p. 480. E. Davenport, " Principles of Breeding," pp. 256-264. 



7 Vernon, " Variation," pp. 229, 296. 



8 Morgan, " Exp. Zool.," p. 266. 



9 Morgan, " Regeneration," p. 9. 



