JOHANSEN 



mammalian forms. The presentation will naturally have to be frag- 

 mentary, and maybe more questions will be asked than answers 

 given. This, I hope, will evoke a vigorous discussion in the distin- 

 guished group of specialists present. Since I have selected the term 

 "physiological phylogeny," I will make no or only superficial refer- 

 ence to the important and intriguing problems related to the onto- 

 genetic development of homeothermy in mammals. 



The paleontologist supplies us with some starting points that 

 may be useful for our discussion. Our knowledge of the origin of 

 mammals as it has been derived from fossils has been supplemented 

 by certain surviving mammals which, in their morphology, indicate 

 an early divergence from the main mammalian stocks. The mono- 

 tremes can, with a fair degree of assurance, be traced back to the 

 early Jurassic period, about 150 million years ago. On this basis, 

 many writers have jumped to the conclusion that the mammalian line 

 became warm blooded earlier than this date, probably as a response 

 to changing climatic conditions, or by being driven by the dominant 

 reptilian stocks to seek life in colder or warmer regions. 



The marsupials, showing striking similarities to the modern 

 opossums, appear next in the fossil record. This indicates to us 

 that the modem marsupials may be representative of the soft part 

 conditions in mammals living 70 to 80 million years ago. A great 

 many of today's marsupial features are, however, to be considered 

 as specialized characters and not truly ancestral conditions. There 

 are about 230 living species of marsupials, found mainly in the 

 Austral- Asian regions, but there are also a few on the American 

 continent. The marsupials show many similarities to placental mam- 

 mals, particularly the Insectivores, which undoubtedly are the oldest 

 stock of placental mammals. 



The placental mammals arose with the Insectivores in the 

 Cretaceous period about 100 million years ago. All present living 

 placental mammals have probably developed from these early 

 Insectivores. The most archiac orders are the present living Insecti- 

 vores, their close relatives, the bats (Ghiroptera), and a diverse 

 group of Xenarthra. The bats, or Ghiroptera, show a very close 

 morphological resemblance to the Insectivores, except of course, 

 the specializations associated with flight. Among the Xenarthra 



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