THE EVOLUTION OF LIFE 



brother, Richard Hertwig, of Munich, has consistently 

 maintained in his admirable Manual of Zoology. 



In opposition to the mechanical ontogeny which I 

 formulated in 1866 and embodied in the biogenetic law, 

 a number of other tendencies in embryology afterwards 

 appeared, and, with the common title of "mechanical 

 embryology," branched out in every direction. The 

 chief of these to attract attention thirty years ago were 

 the pseudo-mechanical theories of Wilhelm His, who 

 has rendered great service to ontogeny by his accurate 

 descriptions and faithful illustrations of vertebrate- 

 embryos, but who has no idea of comparative morphology, 

 and so has framed the most extraordinary theories about 

 the nature of organic development. In his Study of tlie 

 First Sketch of the Vertebrate-body (1868), and many 

 later works, His endeavored to explain the complicated 

 ontogenetic phenomena on direct and simple physical 

 lines by reducing them to elasticity, bending, folding 

 of the embryonic layers, etc., while explicitly rejecting 

 the phylogenetic method; he says that this is "a mere 

 by-way, and quite unnecessary for the explanation o 

 the ontogenetic facts (as direct consequences of physio- 

 logical principles of development)." As a matter of 

 fact, nature rather plays the part of an ingenious tailor 

 in His's pseudo-mechanical and tectogenetic speculations, 

 as I have shown in the third chapter of the Anthro- 

 pogeny. Hence they have been humorously called the 

 "tailor theory." However, they misled a few embry- 

 ologists by opening the way to a direct and purely me- 

 chanical explanation of the complex embryonic phenom- 

 ena. Although they were at first much admired, and 

 immediately afterwards abandoned, they have found a 

 number of supporters lately in various branches of em- 

 bryology. 



The great success tbat modem experimental physiol- 

 ogy achieved by its extensive employment of physical 



383 



