38 ; PROF, J, ARTHUR THOMSON ON 
tion, we may call it—is, in Weismann’s words, that “ the 
power of regeneration possessed by an animal or by a part. 
of an animal is regulated by adaptation to the frequency of 
loss, and to the extent of the damage caused by the loss.” 
It is evident, at once, that the lank and slender bodies 
of worms, the arms of starfishes and brittle stars, the 
sprawling limbs of newts, the long tails of lizards, and so. 
on, are naturally liable to injury, and that a regenerative 
capacity is a quality which natural selection would foster. 
It is also evident that internal organs are much less likely 
to be cut out than external organs are to be cut off. It is. 
also certain that visceral wounds are much more likely to 
be fatal in Vertebrates than in Invertebrates, so that a 
regenerative capacity in the former would be, so to speak,. 
a quality wasted, whereas in the less sensitive Invertebrates, 
where it often occurs, it is very much in demand, 
I do not think we are likely in the present state of 
science to attain to any theory which will enable us to 
express the mechanism of regeneration—how it is that a 
regenerative power resides in a lizard’s tail or in a snail’s. 
horn; it seems more useful, until more data are accumulated, 
to try to come to some decision on the simpler question, 
whether Léssona’s law is a sufficient formula to cover the 
known facts in regard to the occurrence of regeneration, I 
have no desire to dogmatise on so difficult a question ; I only 
say that the hypothesis has considerable merit, and deserves 
to be fairly tested. 
In testing it we are at once led to ask whether there are 
any well-authenticated cases of regeneration of parts such as- 
would not. be likely to be injured or lost in natural 
conditions. A number of difficult cases have been suggested, 
and we may consider a few which are typical of many 
others. 
In his “ Germ-Plasm,” Weismann admitted the difficulty 
of the stork, whose upper beak was accidentally broken off, 
whose lower beak was amputated to the same length, with 
the result that both were regenerated. Was there any 
evidence that such an injury might occur in ordinary life, 
and might therefore have some organic provision made for 
its compensation? The difficulty was lessened by the report. 
