ZOOLOGY AND EOTANY, MICROSCOPY, ETC. 25 
took for his subject ‘ A Half-century of Evolution, with special reference 
to the effects of geological changes on animal life. 5 He discussed the 
various biological movements of the Darwinian era, and summed up 
the present state of evolution theory. The second part of the address 
contaius an interesting and valuable discussion of the geological causes 
of variation and elimination throughout the ages. 
Evolution of the Warm-blooded Animal.* — Mr. H. M. Vernon 
points out that it is now possible to trace with more or less complete- 
ness the various stages by which the lower Invertebrates may have been 
gradually evolved, in a physiological sense, to produce a warm-blooded 
animal, such as man, in which the nervous system appears to possess 
almost perfect power of keeping the temperature of the body constant. 
All cold-blooded animals are not alike in their reaction to tempera- 
ture ; in them, too, may be traced a gradual evolution of the nervous 
control of tissue metabolism, and even a gradual evolution in the reaction 
of tissue-change to temperature in respect of the tissues themselves, 
apart from a special nervous controlling influence. The author shows 
that the respiratory activity of a series of marine animals — from cteno- 
phore to fish — is not by any means equally affected by equal variations 
in temperature. With evolution of structural differentiation, there would 
appear to be a gradual evolution of increased power of resistance to 
variations of temperature ; but perhaps this is rather due to a gradual 
increase in the percentage of solids in the tissues of the organism. 
In many cases the respiratory activity appears to increase regularly 
with the temperature, and there is no evidence that the nervous system 
has any special power of influencing the metabolism ; but this cannot be 
generalised ; since in other cold-blooded animals — earthworms, amphi- 
bians, &c. — the carbonic acid output does not increase regularly with 
increase of temperature. There exist temperature intervals over which 
the metabolism either remains constant or varies but slightly. Experi- 
ments made on frogs and toads cut through the medulla show that this 
want of dependence of metabolism on temperature must be ascribed to 
a direct control of the nervous system over the respiratory activity of 
the tissues.. It is noticeable that the intervals of constant metabolism 
more or less cover such variations of temperature as the animals would 
be ordinarily exposed to under normal conditions, except in the extreme 
cold of winter and extreme heat of summer. 
Since, then, various cold-blooded animals have the nervous system 
sufficiently cooled to exert a controlling power over the tissue meta- 
bolism, the only other conditions necessary to convert these into warm- 
blooded animals of even temperature would appear to be an increased 
heat production by the tissues, coupled with a modification of the external 
covering of the body so as to diminish the heat loss. In some cases 
— bonito, python, lizard — it has been shown that the body temperature 
may be raised considerably above the external temperature. 
The author then considers Sutherland’s interesting observations on 
the variable temperature of monotremes and marsupials. 
The next step in the argument relates to the true placental mammals. 
“ It is generally stated that warm-blooded animals differ from the cold- 
* Science Progress, vii. (1898) pp. 378-94. 
