Introduction 3 



The second or genetic type of adaptive reactions involves selection of genie 

 mutations. There is no evidence that environmental stress per se can induce 

 genie change, but physiological selection of mutations does occur. Those 

 organisms u'hich, by mutation, extend the limits of their physiological lability 

 widen the range of a species. Within a species there are many genie differ- 

 ences, and it is the combined task of the comparative physiologist, ecologist, 

 and geneticist to learn whether an extreme adaptive capacity of certain 

 measured individual animals represents a normal lability for a given gene 

 complex or represents a mutation. This can be learned only by subjecting 

 several generations to the environmental stress. Only those adaptations which 

 have a genetic basis are used in evolution. Extensive combined genetic and 

 physiological studies on animals found at the limits of their normal range are 

 needed. Many problems are implicit in such an approach, for example, the 

 interplay of genetic potentialities with acclimatization capacity of organisms, 

 the factors underlying change in habitat within groups as in movements from 

 water to land, and the basis for annual migration and hibernation of certain 

 kinds of animals. 



Ecological Aspects of Comparative Physiology. Ecology is closely related 

 to phylogeny; where an animal is able to live depends to a considerable extent 

 on where its ancestors lived. Comparative physiology can help to answer such 

 ecological questions as: Why do all echinoderms inhabit the oceans? Why are 

 there practically no marine insects or amphibians? Why are some fish restricted 

 to fast flowing water? How can eels and salmon live in both ocean and fresh 

 water? 



There is no simple way to summarize all of the environmental factors in- 

 fluencing an organism, but we can distinguish four main habitats: marine, 

 fresh-water, terrestrial, and endoparasitic. Each of these can be extensively 

 subdivided, and terrestrial habitats have been most completely classified by 

 zoogeographers and ecologists. 



The comparative physiologist considers the organism in its environment, 

 played upon by a variety of environmental components-^-water, inorganic ions, 

 organic food, oxygen and carbon dioxide, light, high and low frequency 

 mechanical stimuli, gravity, pressure, and temperature. We are concerned 

 with the adaptive responses to these environmental factors. In addition, an 

 animal is influenced in its environment by other organisms-plants and ani- 

 mals. A consideration of the biotic environment leads us beyond the realm 

 of present-dav physiology. Animal behavior will be discussed in this book only 

 as it throws light upon sensory, central nervous, and locomotor mechanisms. 



The characters which compose the bulk of physiological adaptations in 

 different environments are often unstable within an animal group; that is, 

 they show great lability. These are the physiological characters which provide 

 the basis for the ecological distribution of animals. Best examples are found 

 in respiration, nitrogen excretion, osmotic regulation, and circulatory and 

 metabolic responses to temperature extremes. 



Phylogenetic Aspects of Comparative Physiology. The physiology of any 

 group of animals represents a point in the history of that group The com- 

 parative animal physiologist needs, therefore, to know somethmg of phylogeny 

 and taxonomv. The phvlogenist uses the data of paleontology when they are 

 available but' also relies on taxonomy and comparative embryology. During 



