JOHANSEN 



started under fairly constant environmental conditions. It seems 

 also reasonable to assume that the first homeotherms were found 

 among the terrestrial air breathers, because of the advantage of 

 the low conductivity of the air to heat. It is true, of course, that 

 a great many homeothermic animals, whales and seals, for example, 

 are today found in the sea, but they are secondarily aquatic forms 

 with particularly developed insulation. The most stable terrestrial 

 conditions are found and have always been found in the tropics. This 

 justifies the assumption that the first steps toward successful 

 homeothermy were taken in the tropics. 



J. P. Darlington (1948) has advanced excellent arguments telling 

 us that the animal dispersal both for poikilothermic and homeother- 

 mic species started in the tropics and expanded north and south. 

 This expansion and all migratory movements of animals are gen- 

 erally very complex. Thus a successful species with a large dis- 

 tribution range extending north and south can rarely be ascribed 

 to one or a few characteristics. It seems, however, justifiable to 

 assume that a rapid expansion and migration southwards and north- 

 wards from the tropics must have had a bearing on a concomitant 

 establishment of mechanisms for better regulation of internal tem- 

 perature. You will see throughout this discussion, and, I am sure, 

 in Dr. Morrison's paper as well, that the tropics today also have 

 a great number of primitive forms. There we find the monotremes, 

 most of the living marsupials, the overwhelming part of the Chirop- 

 tera, the Insectivores, and practically all the Xenarthra except the 

 nine-banded armadillo. 



Let us then start in the tropics and discuss the qualities known 

 to us of the temperature regulating abilities of some of the primi- 

 tive forms confined to this environment. We can realize that in 

 order to make the transition from the reptilian to the primitive 

 mammalian condition of temperature regulation, a rather radical 

 change in the speed of the biological machinery had to take place, 

 making possible a heat production high enough to keep a maintained 

 high gradient of temperature between the core of the animal and 

 the environment. The assessment of a higher internal temperature 

 will in turn facilitate an accelerated nerve impulse, shortening of 

 the latent period of a musclecontraction, and acceleration of diges- 

 tion, among a number of other biochemical andbiophysically linked 



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