408 -A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



meister's ontogenetic studies, culminating in his brilliant generalizations on 

 alternation of generations. For the first time, the apparently major gap be- 

 tween cryptogamic and phanerogamic plants was effectively bridged by the 

 demonstration of a thread of continuity throughout the plant kingdom, a dis- 

 covery that rendered untenable the concept of separate creation of the different 

 groups. As remarked by Sachs (1890, p. 202) : 



When Darwin's theory was given to the world eight years after Hofmeister's investi- 

 gations, the relations of affinity between the great divisions of the vegetable kingdom 

 were so well established and so patent, that the theory of descent had only to accept 

 what genetic morphology had actually brought to view. 



Darwin's promulgation of the theory of descent, with its explosive effect on 

 the ideas of special creation and constancy of species, was the major scientific 

 milestone of the nineteenth century in so far as the field of taxonomy was con- 

 cerned. Apparent doubts and modifications of the concept of species immut- 

 ability can be gleaned from numerous authors from the time of Adanson and 

 Linnaeus onward. It was not, however, until the full-fledged emergence of the 

 Darwinian thesis in 1859 that there was any major impact on classification. It 

 should not be overlooked that Joseph Hooker was an active party to the develop- 

 ment of the Darwinian theory before it took concrete form, and that he was the 

 chief source of botanical data in support of it. Darwin wrote Hooker in 1845, 

 "I assure you deliberately that I consider all the assistance which you have given 

 me is more than I have received from anyone else, and is beyond valuing in my 

 eyes" (L. Huxley, 1918, p. 492). Hooker's experiences aboard the Erehus in the 

 southern hemisphere, like those of Darwin on the Beagle, turned his interest 

 to a life-long preoccupation with the geographical distribution of plants, notably 

 the occurrence of arctic species in antarctic lands. The subsequent Himalayan 

 travels and the vast familiarity with plants of all parts of the world, acquired 

 as they poured into Kew, made Hooker less dogmatic than most of his contem- 

 poraries in regard to the status of species and genera. He adhered to the tenet 

 of fixity of species as a working hypothesis, however, and remained Darwin's 

 friendly and judicious critic for more than a decade. In 1860 he wrote Harvey : 

 "Eemember that I was aware of Darwin's views fourteen years before I adopted 

 them, and I have done so solely and entirely from independent study of plants 

 themselves" (L. Huxley, 1918, pp. 519-520). Much of the excellence of Darwin's 

 finished presentation may well be ascribed to his constant rewriting of his ideas 

 to meet Hooker's penetrating criticisms. Once the theory of descent was fully 

 formulated, however. Hooker, together with Thomas Huxley, became one of its 

 staunchest early converts and defenders. It was not until the following decade 

 that such men as A. de Candolle, Bentham, and Asa Gray became active ad- 

 herents of the evolutionary conception. Gray wrote A. de Candolle in 1863 : 

 "Well, as to origin of species, you have now gone just about as far as I have, in 

 Darwinian direction, and both of us have been led step by step by the facts 

 and probabilities, and have not jumped at conclusions" (Gray, 1893, p. 498). 

 Gray's critical attitude toward some aspects of the theory of natural selection 

 was not based upon doubts as to the mutability and variation of species, but 

 rather on a religious predilection for "design" in Nature and a skepticism as to 

 whether natural selection could accomplish all that its exponents attributed to it. 



The Darwinian theory provided the long-sought key to the "affinities" recog- 



