302 Plant Life and Evolution 



of living things. The very readiness with which 

 plants respond to stimuli is, however, a source of 

 danger in making sweeping generalizations from 

 insufficient data. Our ignorance of the internal 

 mechanism of the cell, and the fact that often the 

 full effect of a stimulus is not always immediately 

 evident, make it necessary to exercise extreme cau- 

 tion in explaining the real significance of apparently 

 quite obvious reactions to stimuli. 



Another source of error is the too general ap- 

 plication of results drawn from a study of 

 plants to the behavior of animals under like con- 

 ditions. While it is doubtless true that the proto- 

 plasm of plants and animals is, so far as we 

 can judge, very similar in its composition, and in 

 a general way reacts in much the same manner to 

 similar stimuli, it must be remembered that the two 

 kingdoms, plants and animals, have diverged fur- 

 ther and further away from the ancestral organisms, 

 and this divergence has resulted in sharply marked 

 differences, both structural and physiological, so that 

 we cannot safely argue from the behavior of one 

 of the higher plants under certain conditions what 

 would be the result upon an animal subjected to 

 the same conditions. This can perhaps be best 

 shown in considering the questions of reproduction 

 and inheritance. 



Parallel Development of Reproduction in Plants 

 and Animals. Plants and animals show a remarka- 

 ble parallelism in the evolution of the reproductive 



