138 THE RATE OF GROWTH [cs. 



is not to create but to destroy, — to weed, to prune, to cut down 

 and to cast into the fire*. 



Regeneration, or growth and repair. 



The phenomenon of regeneration, or the restoration of lost or 

 amputated parts, is a particular case of growth which deserves 

 separate consideration.. As we are all aware, this property is 

 manifested in a high degree among invertebrates and many cold- 

 blooded vertebrates, diminishing as we ascend the scale, until at 

 length, in the warm-blooded animals, it lessens down to no more 

 than that vis niedicatrix which heals a wound. Ever since the 

 days of Aristotle, and especially since the experiments of Trembley, 

 Reaumur and Spallanzani in the middle of the eighteenth century, 

 the physiologist and the psychologist have ahke recognised that 

 the phenomenon is both perplexing and important. The general 

 phenomenon is amply discussed elsewhere, and we need, only 

 deal with it in its immediate relation to growth f. 



Regeneration, hke growth in other cases, proceeds with a 

 velocity which varies according to a definite law ; the rate varies 

 with the time, and we may study it as velocity and as acceleration. 



Let us take, as an instance, Miss M. L. Durbin's measurements 

 of the rate of regeneration of tadpoles' tails : the rate being here 

 measured in terms, not of mass, but of length, or longitudinal 

 increment t. 



From a number of tadpoles, whose average length was 34-2 mm., 

 their tails being on an average 21-2 mm. long, about half the tail 



* Even after we have so narrowed the scope and sphere of natural selection, 

 it is still hard to understand ; for the causes of extinction are often wellnigh as hard 

 to comprehend as are those of the origin of species. If we assert (as has been 

 lightly done) that Smilodon perished owing to its gigantic tusks, that Teleosaurus 

 was handicapped by its exaggerated snout, or Stegosaurus weighed down by its 

 intolerable load of armour, we may be reminded of other kindred forms to show 

 that similar conditions did not necessarily lead to extermination, or that rapid 

 extinction ensued apart from any such visible or apparent disadvantages. Cf. 

 Lucas, F. A., On Momentum in Variation, Amer. Nat. xh, p. 46, 1907. 



t See Professor T. H. Morgan's Regeneration (316 pp.), 1901 for a full account 

 and copious bibliography. The early experiments on regeneration, by Vallisneri, 

 Reaumur, Bonnet, Trembley, Baster, and others, are epitomised by HaUer, Elem. 

 Physiologiae, vm, p. 156 seq. 



J Journ. Experim. Zool. vii, p. ,397, 1909. 



