56 THE RATE OF GROWTH [ch. 



But this again was clearly present to Haller's mind, and formed 

 an essential part of his embryological doctrine. For he has no 

 sooner treated of incrementum, or celeritas incrementi, than he 

 proceeds to deal with the contributory and complementary pheno- 

 mena of expansion, traction {adfractio)*, and pressure, and the 

 more subtle influences which he denominates vis derivationis et 

 revulsio7iis'\ : these latter being the secondary and correlated 

 effects on growth in one part, brought about, through such 

 changes as are produced (for instance) in the circulation, by the 

 growth of another. 



Let us admit that, on the physiological side, Haller's or His's 

 methods of explanation carry us back but a little way ; yet even 

 this little way is something gained. Nevertheless, I can well 

 remember the harsh criticism, and even contempt, which His's 

 doctrine met with, not merely on the ground that it was inadequate, 

 but because such an explanation was deemed wholly inappropriate, 

 and was utterly disavowed j. Hertwig, for instance, asserted that, 

 in embryology, when we found one embryonic stage preceding 

 another, the existence of the former was, for the embryologist, 

 an all-sufficient "causal explanation" of the latter. "We consider 

 . (he says), that we are studying and explaining a causal relation 

 when we have demonstrated that the gastrula arises by invagina- 

 tion of a blastosphere, or the neural canal by the infolding of a 

 cell plate so as to constitute a tube §." For Hertwig, therefore, as 



* Op. cif. p. 302, " Magnum hoc naturae instrumentum, etiam in corpore 

 animato evolvendo potenter operatur; etc." 



t Ibid. p. 306. "Subtiliora ista, et aliquantum hypotliesi mista, tamen magnum 

 mihi videntur speciem veri habere." 



{ Cf. His, On the Principles of Animal Morphology, Proc. JR. S. E. xv, 

 1888, p. 294: "My own attempts to introduce some elementary mechanical or 

 physiological conceptions into embryology have not generally been agreed to by 

 morphologists. To one it seemed ridiculous to speak of the elasticity of the germinal 

 layers; another thought that, by such considerations, we 'put the cart before 

 the horse ' : and one more recent author states, that we have better things to do 

 in embryology than to discuss tensions of germinal layers and similar questions, 

 since all explanations must of necessity be of a phylogenetic nature. This opposition 

 to the application of the fundamental principles of science to embryological questions 

 would scarcely be intelligible had it not a dogmatic background. No other explana- 

 tion of living forms is allowed than heredity, and any which is founded on another 



basis must be rejected To think that hei'edity will build organic beings 



without mechanical means is a piece of unscientific mysticism." 



§ Hertwig, 0., Zeit und Streitfragen der Biologie, ii, 1897 



