152 THE RATE OF GROWTH [ch. 



as the process rests upon a physical principle, or natural tendency, 

 we may account for its occurrence in a great variety of animals, 

 zoologically dissimilar ; and also for its presence here and absence 

 there, in forms which, though materially different in a physical 

 sense, are zoologically speaking very closely allied. 



Conclusion and Summary. 



But the phenomena of regeneration, like all the other 

 phenomena of growth, soon carry us far afield, and we must draw 

 this brief discussion to a close. 



For the main features which appear to be common to all 

 curves of growth we may hope to have, some day, a physical 

 explanation. In particular we should like to know the meaning 

 of that point of inflection, or abrupt change from an increasing 

 to a decreasing velocity of growth which all our curves, and 

 especially our acceleration curves, demonstrate the existence of, 

 provided only that they include the initial stages of the whole 

 phenomenon: just as we should also like to have a full physical 

 or physiological explanation of the gradually diminishing velocity 

 of growth which follows, and which (though subject to temporary 

 interruption or abeyance) is on the whole characteristic of growth in 

 all cases whatsoever. In short, the characteristic form of the curve 

 of growth in length (or any other linear dimension) is a phenomenon 

 which we are at present unable to explain, but which presents 

 us with a definite and attractive problem for future solution. 

 It would seem evident that the abrupt change in velocity must be 

 due, either to a change in that pressure outwards from within, 

 by which the "forces of growth" make themselves manifest, or 

 to a change in the resistances against which they act, that is to 

 say the tension of the surface ; and this latter force we do not by 

 any means limit to "surface-tension" proper, but may extend to 

 the development of a more or less resistant membrane or "skin," 

 or even to the resistance of fibres or other histological elements, 

 binding the boundary layers to the parts within. I take it that 

 the sudden arrest of velocity is much more hkely to be due to a 

 sudden increase of resistance than to a sudden diminution of 

 internal energies : in other words, I suspect that it is coincident 

 with some notable event of histological differentiation, such as 



