Page:Popular Science Monthly Volume 82.djvu/594

590 Starting out, therefore, with three distinct elements, we find them going through a process of change, in the course of which all three evolve products of very similar properties. If we regard this process of disintegration as in the main a reversal of a process of evolution which once took place, i. e., as a process of devolution, we may say, taking, for instance, RaG, ThD, and Act C as the starting points, that these elements commenced a career of spontaneous change, in the course of which transition products were produced which were quite similar to each other; but ultimately three distinct elements were generated. In the case of the elements, therefore, as in that of organisms, forms which were ultimately to be more or less dissimilar, passed, in the course of their development, through stages in which they closely resembled each other.

It is still more instructive if we consider the stages in the ontogeny of various animals in reverse order. We should then find, taking the Crustacea, for instance, as examples, that starting out with even the most diverse forms of these animals, and imagining them to go through the stages of their development in reverse order, they would grow more and more similar as they approached the larval stage, and when they reached that condition, would be very much alike at corresponding stages of development; just as radium, thorium and actinium are much alike at corresponding stages in their degeneration.

Now, the significance of the embryological phenomena referred to is that these resemblances between animals of quite distinct groups are believed to indicate an ultimate common ancestry for the organisms so related; and since we have observed a condition which we may consider comparable to this among the elements, it seems probable that those radioactive elements which exhibit such close similarities as we have described as their disintegration progresses, originated by evolution from a primary simpler substance; it seems probable, that is, when taken together with the other evidences of evolution herein adduced.

Even if we disregard analogies, the fact that three distinct elements consistently show marked similarity in properties in the course of their disintegration would lead to the presumption that, if we could follow them back far enough, they might prove to be identical. This presumption is strengthened by the analogy we have considered.

It may be remarked that the changes occurring during radioactive disintegration are further similar to those which take place in ontogeny, in that in the former, as in the latter, the various stages are not permanent, but change continuously into other stages; and these changes are in both cases spontaneous, taking place without the aid of any external agency.