REGENERATION OF LOST PARTS IN ANIMALS, 37 
fishes ; it is common among lizards, but not among snakes ; 
and soon. A bird’s toe or the end of a mammal’s tail can 
hardly be regarded as more complex than a starfish’s arm or 
the visceral organs inside a Crinoid calyx ; but while regenera- 
tion is characteristic of Echinoderms, it is at a minimum 
in Birds and Mammals. 
When we follow this fact into greater detail, the in- 
equality of distribution becomes even more striking. The 
weakly developed limbs of Siren and Proteus are not 
regenerated, but the strongly developed limbs of Triton are. 
Although in many families of lizards regeneration of 
the tail is common, it is incomplete or absent in those whose 
tails are used in prehension, as in Chameleons, or as organs 
of defence. A salamander will regrow an amputated limb 
if the bone be cut across and not disarticulated, but in a 
frog the wound heals without regeneration. In Phom- 
chilidium, Loeb finds that a.cut between the second and 
third limbs is followed by regrowth of the oral segments, 
but that is probably unique among Arthropods (Arch. 
Entwickmech., ii. (1895), pp. 250-7. 3 figs.). 
Another general fact is that while the regeneration of 
external parts is common, that of internal parts is rare, 
among backboned animals, and rare even in backboneless 
animals except in cases like that of the earthworm or star- 
fish, where the cutting off of part of the body necessarily 
injures the internal organs, such as alimentary and nervous 
‘systems. If a rabbit’s spleen be removed, it is not replaced ; 
if a fragment be left, it does not grow larger. Similarly the 
excision of a kidney, the thyroid, a liver-lobe, and so on, is 
not known to be followed by regenerative growth. 
Now, the interpretation of these two sets of facts, which 
has occurred to many naturalists, from Réaumur to Weismann, 
is summed up in what is known as Léssona’s law. In 1868 
Léssona formulated the working hypothesis that regenera- 
tion tends to be well-marked in those animals and in those 
parts of animals which are in the course of natural life 
very liable to injury—to which I venture to add two saving 
‘clauses, always provided that the lost part be of some vital 
importance, and that the wound or injury be not in itself 
likely to be fatal. The theory—the Darwinian interpreta- 
