Not] ORGANIC VARIATION. 307 
tion are better understood, for this study is at present nearly 
virgin ground, we will probably be enabled to deduce from it 
criteria, which will offer certain and valuable aid in the study 
of phylogeny. And another important line of investigation 
must become the study of the environment, and of the changes 
of the latter in their influence upon the organism. A con- 
siderable number of experiments have recently been made in 
this line upon the ontogenetic stages of organisms, and a 
smaller number upon the adult organism; and a careful 
analysis of the results of such experiments may give valu- 
able aid in the discrimination of methods for the study of 
variation. 
A final word in regard to regressive development. It is as 
yet an undecided question whether regressive development (or 
the action of Natural Selection during this mode of develop- 
ment) can result in the total disappearance of an organ, or 
whether a structural rudiment must remain. According to our 
conclusions upon the nature of variation, the latter would be the 
more probable view, and forthe following reasons. Forthe action 
of a regressive development, the occurrence of variations below 
- the normal are necessary, so that if these be preserved, a less 
differentiated (z.e. retrogressive) type of organism must be pro- 
duced. The occurrence of variations in regressive development 
is due, just as in progressive, to the physiological exertions 
of the organism to restore that correlation of the organs which 
had been disturbed by a change of environment. Now if the 
change of environment had been a great and sudden one, and 
resulted in a less complex environment, the organism, in order 
to become adapted to the new environment, must produce 
structural variations which are less complex than the normal in 
order to bring about a less intimate and complex correlation of 
the organs than had previously existed. Such variations can 
obviously be produced only as long as the organs continue to 
to be physiologically active ; and necessarily their action must 
cease before the organs disappear. In other words, it is impos- 
sible for variations to lead to the total disappearance of an 
organ, since the very production of variations is dependent 
upon the physiological exertions of the organ. Thus we must 
