WHAT ARE LIVING UNITS? 85 



use of cylinders of pure sulfuretted hydrogen to reinforce the 

 feeble content of some "sulfur springs" may, in careless hands, 

 cause deaths involving serious legal responsibility. 



Some important bodily failures or injuries may not be fatal, 

 providing the deficiencies are made good artificially. Thus 

 diabetics who would otherwise die may be kept alive by regular 

 injections of insulin. In a case recently reported, a pregnant 

 woman, who had for six months remained unconscious and prac- 

 tically helpless following a head injury, gave birth to a normal 

 healthy child. 



In the more restricted sense of "living" mentioned above, sterile, 

 castrated or isolated males or females are said to be alive, though 

 incapable of reproduction. About ten years ago the heath-hen 

 vanished, like the dodo and the great auk, with the death of the 

 sole surviving specimen at Martha's Vineyard. The passenger 

 pigeon is also extinct, though early Americans saw flocks con- 

 taining enormous numbers. On the other hand, given the proper 

 chemical and physical environment, i.e., proper food, milieu, and 

 where necessary, proper mates, some living units may produce 

 with great rapidity. It has been estimated that the ocean would 

 soon be a solid mass of herring, if all the eggs produced developed 

 into mature fish. It is only disease and the constant competition 

 of various forms of life with one another, whereby unrestrained 

 reproduction is counterbalanced, that permit the great diversity 

 and evolutionary development of elementary living units. 



These aspects of the interrelations of living things are con- 

 sidered by the comparatively recently science of ecology, which 

 originated in the study of plants, but has been extended to include 

 animals. The devious nature of these interrelations has been 

 illustrated by the correlation mentioned by Darwin between old 

 maids and red clover. Because of the nature of its flower, red 

 clover is pollinated mainly by bumble bees, which build their 

 nests in the ground. Marauding mice prey upon the nests; but 

 cats, which prey upon the mice, are commonly kept by old maids, 

 who thus unwittingly help the red clover. The curiously shaped 

 flower of the "Dutchman's pipe" (Aristolochia macrophylla) has 

 its pistil well above the stamens in a long corolla, whose narrow 

 mouth contains many stiff hairs pointing inward. A tiny insect 

 can enter, but is held prisoner until it has effected fertilization of 

 the flower; whereupon the hairs droop and the imprisoned insect 



