178 PLANT HYBRIDIZATION BEFORE MENDEL 



vestigations on hybridization move almost exclusively within the pre- 

 viously indicated lines of Kdlreuter. He has especially experimented with 

 the same plant genera in which Kolreuter attained success ; he has in- 

 contestably demonstrated great persistence and restless industry in his 

 numerous experiments, but has scarcely done anything else than to con- 

 firm or carry further the Kolreuter investigations. As rich a source of 

 the knowledge of hybrids as the Gartner work indeed is, one must yet 

 never forget that it must only be used with great caution and critical 

 circumspection." {ib., p. 438.) 



Focke's accurate summary is sufficient as a description of the 

 Gartner memoir. The endeavor has been to present herein the 

 essential facts and observations, as well as the more important 

 conclusions which it contains. In conclusion, however, with due 

 deference to Focke's criticisms, it may be said, attention should be 

 called to what may be considered one of the most fundamental 

 types of expression upon the subject treated. 



The physiological nature of a "species" is stated in the follow- 

 ing sentence : 



"The essentiality of the species, therefore, consists in the definite re- 

 lationship of its sexual powers to other species, which relationship, to- 

 gether with specific form in each species, is a peculiar, special and con- 

 stant one. Form and essence are in this connection one." (2f, p. 163.) 



And again : 



"Not the external similarity in the form and habit of species, but the 

 harmony of the inner nature, gives the capacity for hybrid fertilization : 

 both are likewise not always harmoniously bound together." {ib., p. 186.) 



In this statement is revealed a real comprehension of the phy- 

 siological nature of species ; which comprises something else than 

 the elementary conception of trying-out the crossing of supposed 

 species for the purpose of determining whether their offspring are 

 or are not sterile ; the former case proving the parentage in ques- 

 tion as belonging to different ''species,'' the latter, as being merely 

 "varieties'' of the same species. xAlthough the process may be the 

 same in both cases, the method of presentation above shows a 

 deeper conception of the process involved. 



24. Wichura and the Hybridization of Willows. 



In 1865 appeared Wichura's memoir on the hybridization of 

 plants (5), based upon experiments in the crossing of willows 

 which had occupied him from 1852 to 1858, inclusive. A brief 

 preliminary report had appeared in "Flora" in 1854, and also 

 within the same year in the report of the Schlesische Gesellschaft. 



