ACTIVE RESPONSE OF ORGANISM 347 



Wolff drew from this and other facts the conclusion that the 

 organism possesses a faculty of " primary purposiveness " 

 which cannot have arisen through natural selection. 1 And, 

 as is well known, Driesch derived one of his most powerful 

 arguments in favour of vitalism from the extraordinary 

 regenerative processes shown by Tnbularia and Clavellina. 

 in the course of which the organism actually demolishes and 

 rebuilds a part or the whole of its structure. But under the 

 influence of physiologists like Loeb many workers held fast 

 to materialistic methods and conceptions. 



The great variety of regulative response of which the 

 organism showed itself capable made it very difficult 

 for the morphologist to uphold the generalisations which 

 he had drawn from the facts of normal undisturbed 

 development. The germ-layer theory was found inadequate 

 to the new facts, and many reverted to the older criterion of 

 homology based on destiny rather than origin. The trend 

 of opinion was to reject the ontogenetic criterion of homology, 

 and to refuse any morphological or phylogenetic value to 

 the germ-layers. 2 



The biogenetic law came more and more into disfavour, 

 as the developing organism more and more showed itself 

 to be capable of throwing off the dead-weight of the past, 

 and working out its own salvation upon original and 

 individual lines. 3 A. Giard in particular called attention to 

 a remarkable group of facts which went to show that 

 embryos or larva; of the same or closely allied species might 

 develop in most dissimilar ways according to the conditions 

 in which they found themselves. 4 His classical case of 



1 Beitriige zur Kritik der Darwinschen Lehre, Leipzig, 1898. 



2 See E. B. Wilson, " The Embryological Criterion of Homology," 

 Wood's Holl Biological Lectures, Boston, pp. 101-24, 1895 ; Braem, Biol. 

 Centrblt., xv., 1895 ; T. H. Morgan, Arch. f. Ent.-Mech., xviii. ; J. W. 

 Jenkinson, Mem, Alanchester Lit. Phil. Sot:., 1906, and Vertebrate 

 Embryology, Oxford, 1913; A. Sedgwick, article "Embryology" in 

 Ency. Brtt., p. 318, vol. xi., nth Ed. (1910). 



3 For a detailed treatment of this important point see the 

 remarkable volume of E. Schulz (Petrograd), Prinzipien der rationellen 

 vergleichenden Embryologie. Leipzig, 1910. 



4 " La Pcecilogonie," ;///. Set. France et Belgique, xxxix., pp. 153-87, 

 1905. 



