402 PHYSIOLOGICAL DEVELOPMENT. 



been growing may be reasonably concluded. In a world 

 peopled only by microscopic types there could not have 

 existed the conditions needful for large trees. Gradual dis- 

 integration of rock-surfaces, partly effected by physical 

 agencies and partly by low forms of plants, had to prepare 

 the way for superior plants. The production of sufficient 

 soil by mineralogical decay as well as by the decay of 

 organisms, plant and animal, may be regarded as having 

 been a preliminary to larger plant-growth; and though at 

 present the dependence is far less close than that among 

 animals, yet the benefits yielded to metaphytes by the de- 

 composing actions carried on by protophytes, as well as those 

 carried on by microbes permeating the soil, imply a con- 

 tinued general interdependence throughout the aggregate of 

 plant-forms, apart from more special interdependences. And . 

 then along with this indebtedness of the greater plants to 

 the smaller during the process of evolution, there must be 

 named that indebtedness of plant-life to animal-life which 

 Mr. Darwin has shown in his book on the agency of worms 

 as producers of mould. 



314e. Services of one to another, and consequent unions, 

 of more special kinds are infinitely varied, alike within each 

 kingdom and between the two kingdoms. I refer to those 

 seen in parasitism, commensalism, and other forms of asso- 

 ciation. While they do not conduce to unions of the kind 

 thus far considered, these nevertheless constitute innumer- 

 able links whereby the lives of organisms, plant and animal, 

 are tied together; sometimes for the advantage of both but 

 in most cases for the benefit of one to the injury of the other. 



Among plants the degrees of dependence are various. Un- 

 able to raise themselves into the air and light, some climb, 

 like the ivy, by modified rootlets, or spirally coil themselves, 

 or hang by tendrils. Others there are which gradually 

 strangle the trees they embrace, or which, like lichens in 

 damp climates, festooning the smaller trees, by and by cause 



