biologists to a different point of view. Then the rational 

 idea of the evolution of organic forms explained in a 

 similar rational fashion the observed genetic relationships 

 of groups of plants. No longer did the classifier hesi- 

 tatingly admit the possibility of the evolution of species 

 and deny that of genera and higher groups, no longer did 

 he maintain his artificial groups, which had no more rela- 

 tion to each other than successive throws of dice, but he 

 admitted the whole great scheme implied by the evolution 

 of organic forms from pre-existing types. 



Naturally, it is difficult to point out at just what time 

 the modern trend of botanical work found its origin, but 

 one can say, in a general way, that it was about the middle 

 of the nineteenth century, although of the two criteria of 

 progress to which I shall refer, one dates about a decade 

 before, the other about a decade after that time. The 

 establishment by the botanist Schleiden in 1838, and by 

 the zoologist Schwann in 1839, of the real nature of the 

 cell, and the acceptance of what may be termed the cell 

 doctrine, at once made possible the development of the 

 study of form and structure, both as to adult and as to 

 embryonic organs. With improved optical apparatus 

 and with improved technical methods, many able students 

 added a vast number of demonstrated facts to the general 

 store of knowledge; in fact, for a time the additions to 

 morphological information very much outran the develop- 

 ment of the physiological side, though the latter had had 

 a rational beginning at a prior date. The morphological 

 development depended in the first instance upon the un- 

 derstanding that the cell with its living protoplast, and 

 usually with a wall, constituted a not further divisible 

 morphological unit of living organisms; that every cell 

 must have arisen from a pre-existing one ; and finally, that 

 all but the lower organisms are composed of thous- 

 ands of these cells differentiated into distinct tissues. One 



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