396 De Vries . — On Biastrepsis in its 
costly affair to do this in the case of animals. Moreover, it is 
easy to cultivate plants under quite natural conditions, whilst 
the breeding of rats and mice, or moths or other insects, for 
experimental purposes, can only be carried on under conditions 
which are far from being natural, and which cannot be said to 
be favourable to the normal development of the animals. On 
these and other similar grounds, breeding experiments relating 
to inheritance and variation can be most satisfactorily instituted 
with plants. 
For more than ten years I have been occupied with experi- 
ments of this nature. The object which I have had in view is 
to study the effect of selection under the most favourable 
conditions in producing breeds and varieties, on the one 
hand ; and, on the other, the influence of various external 
conditions upon the production and further development of 
these variations. 
In the course of such experiments the important distinction 
to be drawn between individual and pai'tial variations becomes 
at once apparent. The former are deviations from the type 
which show themselves, though not necessarily in a uniform 
degree, in all the homologous members of the body of an 
individual, as, for instance, a variation in colour of the flowers 
or the fruits. The latter are manifested in but few members ; 
thus only a single leaf or flower may exhibit some special 
peculiarity. I have also found that the partial variations are 
in a much higher degree dependent upon external conditions 
than are the individual variations ; hence the former are more 
useful than the latter as a means of studying the effect of 
external conditions. 
Amongst the partial variations, fasciations and twistings 
of the stem are especially serviceable for this purpose. These 
phenomena used to be regarded as accidental monstrosities, 
but I have shown by breeding experiments that they are 
hereditary 1 . They are typical examples of partial variation ; 
1 Ueber die Erblichkeit der Zwangsdrehungen, Ber. d. deutsch. bot. Ges., 1889, 
vii ; also, Monstruosites hereditaires offertes en echange aux Jardins Botaniques, 
Kruidkundig Jaarboek v. h. Genootschap Dodonaea, Gent, 1897, p. 62. — 
