it will be possible, to find the stage 7//c, which for the present is 
still hypothetical. 
Principally in the same way the probabilitv of the existence of 
the most primitive stage / can be shown. 
This appears from observations I was allowed to make on a ver- 
tebral column in the anatomical institute of Leiden. *) 
On account of the existence of articular planes on the 20% and 
the 21st vertebra it is certain that these vertebrae were provided 
with movable ribs that were missing in the preparation. 
So here 14 dorsal vertebrae are to be found as formula I 
requires. Further we see 5 lumbal vertebrae, the 26th vertebra is 
the last lumbal one, then follows a sacrum, consisting of the 27™ to the 
31st vertebra, as the formula indicates. The caudal vertebrae of the 
preparation are defective, so that we cannot know whether the 35! 
vertebra was the last. The 32"¢ and the 33'¢ vertebrae are extant 
in the preparation, they have however a sacrocaudal form. 
Consequently this vertebral column does not answer entirely to 
formula J, it is a little more primitive and may be regarded as an 
accessory form to a stage /. 
The examples cited show that the accessory forms can likewise 
be explained, if we admit the view, that the various forms are not 
irregular varieties, but the consequences of special processes of devel- 
opment. 
Having this view, we need no longer explain the existence of the 
various forms by the so called variability. This does indeed not give 
an explanation at all, neither does it make us understand that the 
great majority of the vertebral columns forms a morphological 
progression. 
The observations I have made, become however intelligible, if we 
consider that when a species, consisting of many individuals, is in a 
state of phylogenetical development, it would be highly improbable, 
that all the individuals should be transformed with exactly the same 
rapidity. 
If there is, however, a difference of rapidity or intensity of the 
transformation, then it is evident, that, at a given period, in indivi- 
duals living at the same time, very different stages of the process 
of development of the whole species will be represented by groups 
of the individuals. 
And this is what we have seen. 
At the same time it is clear now, why the great majority of the 
1 E. Rosenpera. Ueber eine primitive Form der Wirbelsäule des. Menschen 
Morphol. Jahrbuch Bd XXVII, H. 1. p. 1—118, Tf. LV. 1899. 
