76 HOMOIOTHIJRMISM 



species or individuals which have long lives. Among 

 homoiotherms there is usually a rather sharp demarka- 

 tion between youth and maturity. 



The reasons why animals cease to grow are not cer- 

 tainly known, hut several theories have been advanced. 

 Continued growth is perhaps normal for all living things 

 but it is stopped in some cases by other factors. Di:ffer- 

 entiation may result in the production of such a bulk of 

 inert tissues that growth is no longer possible (Kobert- 

 son, 1923). Growth-stopping substances may increase 

 with age (Beer, 1924). These may be waste products, 

 hormones, or other substances which inhibit cell activity. 

 Metabolic gradients may be concerned (Child, 1915). 

 Eegions where a high rate of metabolism obtains domi- 

 nate those regions of lower rate. When a part gets too 

 far from the dominant region for effective control, 

 growth ceases. There may be a lack of growth-promot- 

 ing substances, such as available food or autocatylist 

 (Kobertson, 1923). 



Growth in homoiotherms is apparently not funda- 

 mentally different from that in poikilotherms, but the 

 adjustments associated with a high degree of somatic 

 differentiation, the prolonged care of young by parents, 

 and the maintenance of constant temperature have appar- 

 ently led to determine growth. The body of a homoio- 

 therm has, through differentiation, come to have a 

 greater segregation of the various functions of its life 

 cycle, and the growth period is rather sharply differen- 

 tiated from the reproductive period. 



Longevity. — Protozoa, as individuals, may live for a 

 few hours, days, or months. Invertebrates exist as indi- 

 viduals for a few months or perhaps even for several 

 years. Anemones have been kept alive for more than 

 60 years and corals have been known to live for 20 to 



