170 



The Journal of Heredity 



cies, nor those which would normally 

 lead to the origin of new species. From 

 this we must conclude that mutations 

 giving rise to new forms in nature may 

 vary in size in the same way and in the 

 same degree. Some of them will pro- 

 duce Jordanian types, but others will 

 give Linnean species. It seems that 

 this is the general rule, though it must 

 be conceded that in exceptional cases 

 the dying out of intermediates may 

 have contributed to the now existing 

 differences. And if we consider the 

 probable connections between the fam- 

 ilies themselves, and their evolution 

 during the primary and secondary geo- 

 logical periods, it is at once clear that 

 the disappearance of linking types must 

 have played more than a subordinate 

 role. 



Therefore we must conclude that, as 

 a rule, or at least in a vast majority of 

 instances, species have been derived 

 from one another by single steps, em- 

 bracing the whole of the differences 

 indicated by the systematical diagnoses. 

 The hereditary factors must change in 

 small groups, say of two or three or 

 some few more units. They must be 

 assumed to be connected with one 

 another, and not as free as the varietal 

 factors, studied in hybridizing, usually 

 prove to be. In this respect it may be 

 of interest to cite the experimental 

 mutations of the species of Oenothera. 

 Here also the factors do not, as a rule, 

 change one by one. but almost always 

 in small groups. Every new type is 

 distinguished from its direct ancestor 

 by quite a number of characters, giving 

 a diagnosis of the same size as those 

 of the wild species themselves. In the 

 beginning these differences have been 

 looked upon as units, afterwards as 

 compound factors, and of late an analy- 

 sis has been attempted, in order to 

 separate the true constituent units. But 

 this study has encountered, until now, 

 the same difficulties as are opposed to 

 a factorial analysis of si)ecific diagnoses 

 in general. In the meantime we must 

 be contented with acknowledging the 



fact that the directly observed muta- 

 tions may, at least in some cases, be 

 of the same nature as must be assumed 

 for the origin of wild species on the 

 basis of Willis' theory. 



The Origin of Genera 



We may go one step further and con- 

 sider the probable origin of genera. 

 Have these been evolved by a series of 

 steps and by the disappearance of the 

 intermediate types? Or is their method 

 of development governed by the same 

 laws as that of the species? It seems 

 probable that at least in many cases 

 the response to the last question must 

 be affirmative, since the assumed dis- 

 ajipearance would include such a vast 

 amount of intermediate types as is 

 hardly compatible with the general 

 laws of distribution. But there is 

 another fact which may be adduced 

 here. I mean the frequency of mono- 

 typic and ditypic genera, including only 

 one or two species. They are very 

 numerous, over thirty-eight per cent of 

 the genera of the world containing only 

 one species each, and about thirteen per 

 cent containing only two. Monotypes 

 and ditypes together constitute, there- 

 fore, more than half of all systematical 

 genera. Genera with three, four or 

 more species than follow in diminish- 

 ing numbers and very large genera are 

 very rare in almost every family. From 

 this consideration we may conclude 

 that there is no real difference between 

 genera and species, except in size, and 

 that therefore the origin of the first 

 has been governed by the same general 

 laws as that of the latter. 



The same analogy that connects Jor- 

 danian with Linnean species is thus 

 connecting species and genera also. 

 The new law, that age goes parallel to 

 area, and that the method of evolution 

 and distribution have been, in the main, 

 the same in all branches of the animal 

 and vegetable kingdom, seems to be the 

 principle which must direct all further 

 researches in the geography as well as 

 in the genealogy of the living world. 



