276 Evolution and Adaptation 
has directed the formation of the new species would not, of 
course, be shown, nor would it make any difference in the 
present connection. 
Before we attempt to reach a conclusion on this point let 
us analyze the facts somewhat more closely. 
In the first place, a number of these cases of discontinuous 
variation are of the nature of abnormalities. The appearance 
of extra fingers or toes in man and other mammals is an ex- 
ample of this sort. This abnormality is, if inherited at all, 
inherited completely ; that is, if present the extra digit is per- 
fect, and never appears in an intermediate condition, even 
when one of the parents was without it. The most obvious 
interpretation of this fact is that when the material out of 
which the fingers are to develop is divided up, or separated 
into its component parts, one more part than usual is laid 
down. Similarly, when a flower belonging to the triradiate 
type gives rise to a quadriradiate form,—as sometimes 
occurs, — the new variation seems to depend simply on the 
material being subdivided once more than usual; perhaps 
because a little more of it is present, or because it has a 
somewhat different shape. My reasons for making a sur- 
mise of this sort are based on certain experimental facts in 
connection with the regeneration of animals. It has been 
shown in several cases that it is possible to produce more 
than the normal number of parts by simply dividing the ma- 
terial so that each part becomes more or less a new whole, 
and the total number of parts into which the material becomes 
subdivided is increased. It seems not improbable that phe- 
nomena of this sort have occurred in the course of evolution, 
although it is, of course, possible that those characters that 
define species do not belong to this class of variation. To 
take an example. There are nine neck-vertebre in some 
birds, but in the swan the number is twenty-five. We cannot 
suppose that the ancestor of the swan gradually added enough 
materially to make up one new vertebra and then another, 
