THE MANIFESTATIONS OF LIFE 41 



zone, and the general variation of fauna and flora as we 

 see it affected by latitude and altitude. 



In the eternal winter at the earth's poles, and in the 

 eternal snows of high mountain altitudes there is no 

 life; on the lowlands near the earth's equator, where it is 

 perpetual summer, life, both vegetable and animal, is 

 most abundant and most active. 



The most striking and most beautiful example of 

 thermotropic irritability is to be seen, however, in the 

 alternating summer and winter of the temperate zones. 



During the winter months when the waters are locked 

 in ice and the ground covered with snow, the landscape 

 presents an appearance of desolation suggesting uni- 

 versal death. The plants seem dead, the trees lifeless 

 skeletons; the insects and smaller animals have disap- 

 peared; the migratory birds have flown, and only the ever- 

 green trees, and most hardy birds and mammals remain. 

 If the winter be exceptionally severe, many of these 

 may be killed. As the position of the earth changes and 

 the days lengthen and the warmth of the sun's rays 

 strengthens, the conditions change. The snow and 

 ice melt, and in the warm moist earth the roots and 

 seeds swell and the tender grass and plants emerge. The 

 trees soon spread their leafy canopies, the flowers bloom, 

 the insects leave their hiding places, the birds return, 

 the animals creep from their shelters, and the latent 

 invisible life once more returns to its state of activity 

 and vigor. But see how rapid is the accelerating in- 

 fluence of the increasing temperature in all these changes. 

 Upon a frosty spring morning in the country, one notes 

 the greening grass and the flower buds nestling in the 

 sheltered places; an occasional insect clings feebly to 

 the stem of a plant or crawl upon the ground, easily 

 picked up, benumbed and stiff; perhaps a snake has 

 curled upon a stone or stump to catch the morning sun, 

 so stiff and sluggish as to fail to get away in time to 

 avoid capture. Yet, by afternoon, the magic of the sun's 

 warmth has effected a striking change: the grass is 



