174 GÖTE TL'RESSON 



tion. To transfer the species-concept to the pure hne-concept on 

 account of the constancy of the genotype, as is done by Lotsy, is at 

 the same time to ignore the ecological side of the species problem, no 

 matter what ideas LiNN.tius had as to the cause of the diversity within 

 his species. These latter represent from our point of view, as stated 

 above, ecological units of extraordinary importance. Thanks to its 

 genetically heterogeneous nature the Linnean species is able to cover 

 a vast region by responding genotypically to a wide range of different 

 habitats within the region. It is by studying the phenomena of these 

 responses and their resulting products, the ecotypes, that we should gain 

 a knowledge of the origin of the genecological units. 



A point of particular interest of genecology is afforded by the 

 behaviour of species hybrids in nature. When it is found by experi- 

 ment that individuals belonging to different Linnean species may be 

 crossed and give — at least to a certain extent — fertile offspring, the 

 question as to the causes of the rarity of such species hybrids in nature 

 becomes acute. The distributional peculiarities of these hybrids, viz. 

 their localization at isolated points within the region covered by the 

 two species, their sporadic occurrence between the distribution areas 

 of the two parent species, the tendency of certain species hybrids to 

 increase when nature is disturbed by man, etc., furnish additional and 

 important points of attack for genecological inquiry. A closer study 

 of these and related phenomena will no doubt tend to emphasize the 

 view that the treatment of the species problem along the lines here 

 advocated is urgently needed as a complement to the Mendelian study 

 of the species problem, if a deeper understanding of the questions in- 

 volved is to be attained. 



The practical difficulties to be overcome in the study of the 

 grouping in nature of individuals into genecological units are rather 

 great. The ground needed for representative collections from different 

 habitats and from different regional points within the distribution area 

 of the species to be investigated is considerable, and the care of such 

 extensive cultures is by no means an easy matter. It may be mentioned 

 as an example that the writer's genecological work on Hieracium iim- 

 beUatum has necessitated the transplantation and the cultivation in per- 

 manent cultures of more than 1200 individuals. A full knowledge of 

 the hereditary habitat types of this species in Scandinavia could ne- 

 vertheless only be drawn from still greater material. It is sincerely to 

 be hoped that experimental gardens for wild plants similar to that 

 started at this Institute will be arranged in other countries, where 



