338 MR. W. WOODLAND ON THE [ Apr. 21, 
connection with respiration cannot have constituted its primitive 
raison a étre. 
Thus, observing the necessity for the development of the dia- 
phragm, we find that its variations in disposition and contour 
found in the Struthiones and Mammalian orders fully confirm 
the conclusion deduced on a@ priori grounds. 
Another organ which, though not peculiar to the Mammalia, 
is yet a diagnostic feature of terrestrial vertebrates, is the meta- 
nephric kidney, and that there is possibly a relationship between 
terrestrial modes of lecomotion and the evolution of this organ 
T will now endeavour to show. 
It is well established that the metanephros is a development 
of the definitive mesonephros which has lost its nephrostomata, 
acquired a separate duct, and become more or less concentrated 
in form. It has been pointed out above that concentration 
of structure is an essential concomitant of that increase of 
impulsiveness which results from the increased activity of the 
animal under conditions which involve marked reactions between 
the body and the medium or substratum. That this is so in the 
case of the kidney, it is only necessary to compare the elongated 
mesonephric bodies of Pisces and aquatic Urodeles with the same 
organs of the terrestrial Amphibia and the metanephric bodies 
ot Reptilia, and again to compare these latter with their repre- 
sentatives in Mammalia, in which both concentration of structure 
and impulsiveness of locomotion attain their maxima. With 
regard to the loss of nephrostomata, the same relation holds. 
In all Pisces, with the exception of certain Elasmobranchii, the 
mesonephric bodies retain connection with the ccelom by means 
of the nephrostomial tubules. Whether the absence of these 
fragile structures in Elasmobranchs (their function with regard 
to the celom perhaps being assumed by the abdominal pores) is 
to be attributed to the fact that these fishes either are or are not 
descended from the most active members of their class (and it 
must be remembered that they are surface forms) is not certain, 
but it is possible. In the aquatic Urodeles the nephrostomes are 
present, but in the terrestrial Anura they have completely lost 
their connection with the celom (which latter, as in Elasmo- 
branchs, has had to discover another means of exit for its waste 
products). The causal relation between disruption of the nephro- 
stomata and the adoption of a terrestrial life is here clearly shown 
by the ontogeny. In the development of the frog, the meso- 
nephros is at first in communication with the coclom: by means of 
the nephrostomata, but at the period of metamorphosis this con- 
nection is severed, and the kidney, becoming more concentrated, 
finally assumes the definite form of the adult : structure. Needless 
to say, the metanephros of Reptilia, Aves, and Mammalia is 
totally devoid of nephrostomes, that of the last not possessing 
them at any stage of development. The division of the meso- 
nephros into two portions, one coming into relation with the 
testes, and the other—the definitive mesonephros—yretaining its 
