466 KINSHIP AND ADAPTATION 
varied sets of conditions offering fresh opportunities for 
living things. 
Life as we know it is possible only below a certain tempera- 
ture. The greatest heat in which living things are found to 
grow is that of certain hot springs where, it is reported that 
a centigrade thermometer registers about 55° (equivalent 
to 131° Fahrenheit). It will be remembered that water 
scalds at about 60° C. or 140° F. Under these extraordinary 
conditions, certain microscopic plants of most simple organi- 
zations are found to thrive.! It is fair to assume therefore 
that living creatures could not have appeared upon the earth 
until the crust had so far cooled that the waters were con- 
siderably below their boiling-point. Since the simplest forms 
of life we know and the oldest fossils we have, are aquatic, 
it is probable that the first living things appeared in the water; 
and since all the animals we know depend directly or in- 
directly upon vegetable food, it seems most likely that the 
earliest organisms were plants and that from them animals 
evolved. 
Confining our view to the vegetable kingdom, which here 
chiefly concerns us, we may picture to ourselves its evolution 
as proceeding in a general way from plants of comparatively 
simple organization, to those whose structure is more and 
more complex, greater morphologicai differentiation accom- 
panying fuller physiological division of labor. Such increase 
in complexity we speak of as progress from lower to higher 
organization, without meaning to imply that the higher 
forms are any more perfectly adapted than the lower to their 
respective environments. Indeed the simpler forms may be 
so well adapted to the less trying conditions that they may 
persist through countless generations essentially unchanged, 
provided thay have the opportunity to live in the kind 
of environment which suits them. Thus we find to-day, 
growing in water, plants which may be fairly supposed to 
have retained the main features characteristic of the pro- 
genitors of the vegetable kingdom. 
1 Experiment shows that the spores of other very simple plants are 
not killed by a temperature considerably above that of boiling water, 
but they cannot grow under such conditions. 
