A further examination of the facts throws a certain amount of light on the relation between the processes of self-division and of regeneration. The following questions may serve to guide us in our examination:—
(i) Is regeneration found only in those groups in which self-division takes place as a means of propagation; or, conversely, does self-division only occur in those groups that have the power of regeneration?
(ii) Is regeneration confined, in the groups that make use of self-division as a means of propagation, to those regions of the body where the self-division takes place?
(iii) Is regeneration as extensive in the groups that do not propagate by self-division as in those that do?
(iv) Can we account, in any way, for the presence of self-division in certain groups, and for its absence in others?
(v) What relation exists between the forms that prepare for subsequent self-division and those that do not?
The first question is easily answered. Regeneration is also found in nearly all the other groups that do not propagate by self-division,—as, for instance, the mollusks, vertebrates, etc. The second half of the question may also be answered. All the groups that propagate by self-division have also the power of regeneration.[69]
In answer to the second question there is ample evidence showing that regeneration is by no means confined to those regions of the body in which the self-division occurs.
In answer to the third question, it may be stated that although, in the groups that propagate by self-division, regeneration may be present in nearly all parts of the body, the same phenomenon occurs in other groups that do not propagate by division.
The fourth question offers many difficulties, and our answer will depend largely upon what we mean by “accounting for” the process in certain groups. If the question is interpreted to ask, Why does an animal divide? no answer can be given. If it is meant to ask, Can we see how the process would be difficult, or even impossible, in certain groups and not in others? then an approximate answer may be given, or at least an hypothesis formed. In the first place, the power of regeneration must be present in the region at which the self-division takes place in order that the result may lead to the formation of new individuals, or else be acquired in that region along with the acquirement of the means for division. A leech is not much more complicated than a marine annelid, yet it has little or no power of regeneration; hence, perhaps, propagation by division could not be acquired by the leeches until they had first acquired the power to regenerate. In the second place, in certain forms a separation of the body into two parts would lead to the death of one or of both parts, owing to the dependence of the different regions upon each other. In forms like the vertebrates, insects, crustacea, etc., we can readily see why this would be the case. Hence propagation by means of self-division could not be acquired, since the division itself would lead to the destruction of the organism. In the third place, the structure of the body may be such that the process of self-division would be mechanically impossible. A hard outer coat, like that of the sea-urchin, combined with a weak development of the musculature of the body, would effectively prevent the self-division of the animal.