152 Changing Plant and Animal Nature 



lies the adaptation that the butterfly does not emerge from 

 her winter sleep until the leaves are ready for her. These 

 details are all tied up with the change of temperature which 

 makes possible the movement of water in the soil, into the 

 roots of the tree, up the stem, and into the expanding buds. 

 It is the same rise in temperature — and liberation of water 

 — that forces the butterfly out of the pupa case and the 

 leaves out of their buds. It is the same rise in temperature 

 that brings the frogs and toads out of their winter sleep and 

 stimulates or releases the millions of seeds to sprout. And 

 the same change in temperature releases myriads of one- 

 celled plants and animals lying dormant in the water. 



The Interdependence of Living Things 



These many plants and animals that make their shy ap- 

 pearance with the coming on of warmer weather are all 

 interrelated in the sense that they depend upon one another 

 for their existence. The green plants alone are capable of 

 manufacturing usable food from the abundant raw ma- 

 terials — water, carbon dioxid, and the soluble salts of the 

 soil, which are present in natural waters. These plants in 

 turn, beside serving as food for the animals and for those 

 plants like fungi and bacteria which cannot produce their 

 own, depend upon the animals and upon the fungi and bac- 

 teria for two essential processes. It is these other organisms 

 that in their metabolism constantly restore to the atmos- 

 phere the carbon dioxid which is essential for the life of the 

 green plants. And it is these other organisms that restore 

 constantly to the soil and the water the materials which the 

 green plants build into their bodies and which would eventu- 

 ally be exhausted but for the restoring processes of decay 

 and destruction. It is conceivable that a primitive type of 

 bacterium capable of nourishing itself with the aid of sun- 

 light from the inorganic substances of the ocean might con- 

 tinue to exist over an indefinite period in utter independence 

 of other living things. Such an independent existence, how- 



