390 TRANSACTIONS OF SECTION D. 
ment took place. It is, however, inevitable that, as phylogenetic evolution pro- 
gresses, the conditions under which the young organism develops should change. 
In the first place, the mere tendency to acceleration of development, to which 
we have already referred, must tend to dislocate the correlation between the 
ontogenetic series and the environmental series. Something of this kind seems 
to have taken place in the life-cycle of many Hydrozoa, resulting in the sup- 
pression of the free medusoid generation and the gradual degeneration of the 
gonophore. But it is probably in most cases change in the environment of the 
adult that is responsible for such dislocation. 
To return to the case of the amphibians. At the present day some amphi- 
bians, such as the newts and frogs, still lay their eggs in water, while the 
closely related salamanders retain them in the oviducts until they have developed 
into highly organised aquatic larvie, or even into what is practically the adult 
condition. Kammerer has shown that the period at which the young are born 
can be varied by changing the environment of the parent. In the absence of 
water the normally aquatic larve of the spotted salamander may be retained in 
the oviduct until they have lost their gills, and they are then born in the fully- 
developed condition, while, conversely, the alpine salamander, whose young are 
normally born in the fully-developed state, without gills, may be made to deposit 
them prematurely in water in the larval, gill-bearing condition. 
There can be no doubt that the ancestral amphibians laid their eggs in water 
in a completely undeveloped condition. The habit of retaining them in the body 
during their development must have arisen very gradually in the phylogenetic 
history of the salamanders, the period for which the young were retained 
growing gradually longer and longer. It is obvious that this change of habit 
involves a corresponding change in the environmental conditions under which 
the young develop, and in cases in which the young are not born until they 
have reached practically the adult condition this change directly affects 
practically the whole ontogeny. We may say that the series 
iH, E, E, : f : : ; ‘ 5 En has become 
Ey’ E,' E,’ : : : é 3 2 é En’, 
and as the change of environment must produce its effect upon the developing 
organism the series 
M, M, M,; , , : : : ‘ Mz will have become 
M,’ M,’ M,’. : 4 ; : : M”’. 
We must remember that throughout the whole course of phylogenetic evolu- 
tion this series is constantly lengthening, so that what was the adult condition 
at one time becomes an embryonic stage in future generations, and that the 
series thus represents not only the ontogeny, but also, though in a more or less 
imperfect manner, the phylogeny of the organism. 
The character of each stage in ontogeny must depend upon (1) the morpho- 
logical and physiological constitution of the preceding stage, and (2) the nature 
of the environment in which development is taking place. We cannot, however, 
distinguish sharply between those two sets of factors, for, in a certain sense, 
the environment gradually becomes incorporated in the organism itself as 
development proceeds, each part contributing to the environment of all the 
remainder, and the influence of this internal portion of the environment ever 
becoming more and more important. 
The whole process of evolution depends upon changes of environment taking 
place so gradually that the necessary self-adjustment of the organism at every 
stage is possible. In the case of our amphibia the eggs could probably undergo 
the first stages.of development, the preliminary segmentation, within the oviduct 
of the parent just as well as in the water, for in both cases they would be 
enclosed in their envelopes, and the morphological differences between the early 
stages in the two cases might be expected to be quite insignificant. But it 
must be the same at each term of the series, for each term is built upon the 
foundation of the preceding one, and the whole process takes place by slow and 
imperceptible degrees. ; . 
It is true that by the time we reach the formation of the vestigial gill-slits 
in the embryo of one of the higher vertebrates the environmental conditions are 
