Regeneration / by Thomas Hunt Morgan.

  • Thomas Hunt Morgan
Date:
1901
Catalogue details

Licence: In copyright

Credit: Regeneration / by Thomas Hunt Morgan. Source: Wellcome Collection.

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    ingcase in favor of this view is that the terrestrial salamander, accord- ing to Lessona, cannot reproduce lost parts, whilst another species of the same genus, the aquatic salamander, has extraordinary powers of regrowth, as we have just seen; and this animal is eminently liable to have its limbs, tail, eyes, and jaws bitten off by other tritons.” Lang, referring to the brittleness of the tails of lizards, points out that this is a very useful character, since the bird of prey that has struck at a lizard gets hold of only the last part of the animal to dis- appear under cover; the lizard escapes by breaking off its tail. The brittleness of the tail is, therefore, an adaptive character that has become fixed by long inheritance. To this example may be added that of certain land snails in the Philippine Islands. The individuals of the genus helicarion live on trees in damp forests, often in great droves. They are very active, and creep with unusual swiftness over the stems and leaves of the trees. Semper has recorded that all the species observed by him have the remarkable power of breaking off the tail (foot) close behind the shell, if the tail is roughly grasped. A convulsive movement is made until the tail comes off, and the snail drops to the ground, where it is concealed by the leaves. Semper adds that in this way the snails often escaped from him and from his collectors, leaving nothing behind but their tails. The tail is said to be the most obvious part of the animal, and it is assumed that this is, therefore, the part that a rep- tile or bird would first attack.^ Lang states that in this case external influences have produced an extraordinarily well-developed sensitive- ness in the animal, so that it reacts to the external stimulus by volun- tarily throwing off the tail. It would be, of course, of small advantage to be able to throw off the tail unless the power of regenerating the lost organ existed, or was acquired at the same time as the extreme sensitiveness that brings about the reaction. Lang does not state, however, explicitly that he believes the regenerative power to have arisen through the exposure of the tail of the lizard and the tail of the snail to injury, although he thinks that the mechanism by means of which these parts are thrown off has been acquired in this way. Several other writers have, however, used these same cases to illus- trate the supposed principle of liability to injury and power of regeneration. Weismann in his book on The Germ Plasm has adopted the principle of a connection between regeneration and liability to in- jury and has carried it much farther than other writers. We can, therefore, most profitably make a careful examination of Weismann’s 1 Whether, having once failed in this way to obtain the snail, the bird or lizard would not learn to make a frontal attack is not stated. Or shall we assume that the tail is all that is wanted?
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