A CRITICAL RE-STATEMENT OF THE BIOGENETIC LAW. 91 
outside the purview ,o£ the present communication, the slightest survey oi: 
Vertebrate evolution shows a series of triumphs over limiting environments 
of medium, temperature, space and time which has been based as much on the 
substitution of new for old organs as on the continuous elaboration of 
particular ones. When a given organ is wholly transformed in the course 
of evolution, it rarely shows traces in ontogeny of the original steps of 
its transformation {e.g. bony scales to fin-rays, horny scales to feathers, 
lobate fins to pentadactyle limbs, pentadactyle limbs to wings). The final 
form alone is inherited and develops directly. But when originally separate 
organs are ultimately united into one organ, some stages of the process of 
amalgamation are necessarily repeated (e.g. branchial arches to hyoid, 
vertebral elements to vertebras, muscle-buds for paired limbs, anchylosis of 
limb-bones, segmental tubules of kidney, &c.) . And when a new organ has 
arisen in intimate dependence on an old organ, the old organ maj4 still 
remain necessary for the development of the new (Kleinenberg, 1886). 
Thus backbone replaces notochord, and bone replaces cartilage in present as, 
doubtless, in past ontogeny, for the former organ or tissue is still necessary 
as scaffolding for the later one ; and the constant development of gill-slits in 
the ontogeny of terrestrial Vertebrates is but another illustration of the same 
phenomenon, as Sedgwick has already pointed out (1894) — for a complex 
double circulation that has been elaborated along channels determined by a 
branchial circulation cannot readily depart from the phyletic steps of its 
formation. It is this formative dependence of one organ, or set of organs, 
on another that confers on Vertebrate ontogeny its marked recapitulative 
character. 
13. But it is equally clear that the whole succession is explicable without 
recourse to the theory of successive adult incorporations, and that the onto- 
genetic stages afford not the slightest evidence oi the specially adult features 
of the ancestry. So far as notochord and gill-slits are concerned, they make 
their appearance in the earliest larval stages of every animal that presents 
them, including Amphioxus itself. Their phyletic origin is still wrapped in 
obscurity. The case is hardly different as regards cartilage, bone, scales, 
feathers, hairs, lungs, limbs, and all the other organs concerned. No example 
can be adduced of any of these organs arising in an adult stage of ontogeny. 
Until that evidence is produced, it is idle to claim that recapitulation which 
involves any of these organs is a repetition of specifically adult ancestral 
features. Moreover, it is impossible to overlook the fact that some of the 
most pregnant changes in the characteristics of the higher Vertebrates are 
directly or indirectly traceable to changes in the earliest stages of the 
ontogeny. The elaboration of the brain in Birds and Mammals, and the 
development of their social and aesthetic senses, are connected with the inter- 
polation of the helpless chick, puppy, or baby stage in the ontogeny, which 
from the simplest beginnings has led to the development of educability and 
