428 GYMNOSPERMS 



factors may have a great influence upon the size and quality of the 

 products, but they are still pumpkins and corn. 



The apple, pear, and quince resemble each other rather closely, 

 and we believe that their structural similarities are due to genetic 

 relationship; in the same way, the cherry, peach, and plum have a 

 genetic relationship which is indicated by similar structures. In each 

 group the three members have come from a common ancestor in the 

 remote past; and, farther back, these two common ancestors may 

 have come from some still more remote ancestor. 



The morphologist stresses mutation, smaller variations, hybridiz- 

 ing, and natural selection in accounting for changes, and he is too 

 likely to overlook the influence of soil, climate, and other conditions. 



The aquatic form of Polygonum amphibiiim differs so much from 

 the land form that they were described as two species. How far is 

 the capacity for such immediate responses inheritable? Would 

 Proserpinaca paluslris, grown in a mesophytic habitat for a thousand 

 generations, lose the capacity for producing dissected leaves when 

 grown in water? 



Most botanists believe that the land flora came from an aquatic 

 ancestry. When algae transmigrated to the land it became necessary 

 to develop protective, conducting, and supporting structures. The 

 theories of Church,'" as expressed in his Thallassio phyla, the 

 views of Scott, as set forth in his Extinct Plants^^^ and in the third 

 edition of his Studies in Fossil Botany,^^^ and also the views of 

 Seward, as presented in his Fossil Plants^'^'' and in his Plant Life 

 through the Ages,^"^^ are suggestive. 



Let us suppose that some algae became stranded. Those which 

 did not die developed conducting cells, at first merely elongated, 

 then with thickenings and with groupings into bundles. Later, more 

 algae transmigrated, and, having the same conditions to cope with, 

 developed elongated cells with thickenings and groupings like the 

 previous transmigrants. The modern botanist, finding the two 

 descendents resembling each other, assumes that they are related. 



Paleobotanists are often entirely dependent upon such evidence, 

 because the wood may be the only part preserved; but morpholo- 

 gists, especially anatomists, lay no less stress upon the evidence of 

 anatomy. 



