idea into the theory that each individual recapitulates in his development 

 the history of his race. The stages are supposed to represent all the types of 

 his ancestors. In a general way this is true only as a restatement of von 

 Baer's law. But, strictly speaking, it is not true, for example, that you once 

 passed through a hydra stage or a fish stage. All we can say is that each of 

 us has passed through stages which resemble corresponding stages in many 

 classes of animals (see illustration, p. 459). 



What Brings About Differentiations during Development? 



Conditions for Development External conditions influence the de- 

 velopment of organisms, just as they influence growth or metabolism in 

 general. Thus plants growing in northern regions, with long days and 

 short nights, during the summer, mature more rapidly than those grown 

 from the same stock in regions having shorter days (see pp. 251-252). The 

 submerged leaves of certain plants are quite unlike those growing above 

 the surface of the water (see illustration, p. 203). 



Temperature influences development in many ways, sometimes very 

 strikingly. The eggs of frogs will develop into tadpoles very much more 

 rapidly in warm water than in cold. Jacques Loeb showed that by chang- 

 ing the temperature it is possible to modify the rate of development and the 

 life-duration of animals. Fruit flies, for example, live about eight weeks, 

 from the Qgg to the end of adult life, at ordinary room temperature. At 

 the temperature of a warm summer day (about 86° F), all their life proc- 

 esses are so speeded up that development is completed in three weeks. By 

 lowering the temperature to 50° F we can retard all the life processes of the 

 insect and stretch its life to nearly six months. 



Temperature influences various aspects of metabolism and various tissues 

 in different ways. Some species of butterflies and moths produce two broods 

 a year, surviving the winter in the pupal stage. The spring form is often 

 strikingly different from the late summer form in size and pigmentation. 

 Experiments indicate that so-called local races or varieties of insects differ 

 from one another chiefly, if not entirely, because of temperature. 



Chemical Influences We can most easily observe the influence of 

 chemical substances upon growth and development in the lower forms. 

 But more complex forms also show modifications. Mollusks, crustaceans, 

 and other animals have apparently become modified under natural condi- 

 tions in which sea water is sometimes diluted by rains or concentrated by 

 evaporation (see illustration, p. 359). Professor Charles R. Stockard (1879- 

 1939), of the Cornell medical school, brought about amazing changes in 

 development of the minnow Fundulus by changing the chemical composi 

 tion of the sea water (see illustration, p. 360). 



357 



