466 KINSHIP AND ADAl^TATION 



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 1H1° Fahrenheit). It will be remembered that water 

 scalds at about 60° ('. 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- 

 siderabljr 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 vegctaljle food, it seems most likely that the 

 earliest organisms were plants and that from them animals 

 evolved. 



Confining our view to the vegetable kingdom, whicli 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 morphological 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 imjily that the higher 

 forms are anj' more ])erfectly adapted than th(> lijwer to thnv 

 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 opjjortunity to liv(.> in the kind 

 of environment which suits them. Tluis w(> find to-day, 

 growing in water, plants which may be fairly supposed to 

 have retained the main features characteristic of the pro- 

 genitors of th(! vegetable kingtlom. 



' Exporimont shows IIkiI llic siMircs of odicr \'('ry simple plants are 

 not killed by a icnipi r:il tire considerably aliove that of boiling waler, 

 bill llie\' i-ainiol L^row under sueli eondil ions. 



