300 THE LIFE OF THE PLANT 



out of which other groups are formed, was called by 

 Linnaeus a species. Out of species the genus is formed, 

 out of genera families, and so on. E.g. a violet and a 

 heart's-ease represent, according to Linnaeus, two species 

 of the genus Viola ; two poplars the black and the 

 white are two species of the genus Populus ; a donkey 

 and a horse fall into the same genus Equus, the wolf and 

 the dog into the genus Canis, and so on. The determina- 

 tion of the groups called species marked a great advance 

 in science : it rendered possible the strictly systematic 

 classification of organisms. However, after having 

 established this collective unit of their system, this 

 group the species, the systematists, not so much Linnaeus 

 himself as his followers, declared that a species is some- 

 thing actually stable, invariable in space or time ; that 

 species have always been and will ever be what they are 

 at present ; that the transformation of one species into 

 another is out of the question altogether, and hence that 

 the theory of the common origin of all organisms is quite 

 inadmissible. So far we have been discussing data which 

 have been gathered by the theory of metamorphosis, 

 by comparative morphology, embryology, and palaeon- 

 tology, and which testify to the possibility of the transi- 

 tion of forms of one family into those of another (e.g. from 

 Boragineae into Labiatae), the possibility of transition 

 from a spore to a seed-plant, the impossibility of finding 

 any line of demarcation between the vegetable and the 

 animal world, etc. But what can be the significance of 

 all these facts if it is true that no transition is possible 

 in the case of beings most closely related, in the case of 

 species of the same genus ? If the violet and the 

 heart's-ease have always been so different from each 

 other, if they are unable to vary, if species are immutable, 

 then certainly all our considerations as to transition 

 from one family to another, from one order to another in 

 the vegetable kingdom, as also from one kindgom to 

 another, are futile. Hence it is clear that the problem 



