TRANSACTIONS OF SECTION D. 841 
efferent vessels arise in each arch independently and almost simultaneously: the 
_ afferent vessel soon acquires connection with the heart; but, unlike 22. esculenta, 
_ the efferent vessel has no connection with the heart until the gills are about to 
atrophy. 
In other words, the continuous aortic arch, from heart to aorta, is present in 
R. esculenta prior to the development of the gills: it becomes interrupted while the 
gills are in functional use, but is re-established when these begin to atrophy. In 
. temporaria, on the other hand, there is no continuous aortic arch until the gills 
begin to atrophy. 
The difference is an important one, for it is a matter of considerable morpho- 
logical interest to determine whether the continuous aortic arch is primitive for 
vertebrates: z.e., whether it existed prior to the development of gills. This point 
could be practically settled if we could decide which of the two frogs, R. esculenta 
and R&. temporaria, has most correctly preserved its ancestral history in this 
respect. 
Hit this there can be little doubt. The development of the vessels 
in the newts, a less modified group than the frogs, agrees with that of 2. esculenta, 
and interesting confirmation is afforded by a single aberrant specimen of R. 
temporaria, in which Mr. Bles and myself found the vessels developing after the 
type cf 2. esculenta, t.e.,in which a complete aortic arch was present before the 
gills were formed. 
We are therefore justified in concluding that, as regards the development of 
the branchial bloodvessels, . esculenta has retained a primitive ancestral 
character which is lost in &. temporaria, and it is interesting to note that were 
our knowledge of the development of amphibians confined to the common frog, the 
most likely form to be studied, we should, in all probability, have been led to 
wrong conclusions concerning the ancestral condition of the bloodvessels in a 
point of considerable importance. 
ee Pe 
A matter which at present is attracting much attention is the question of 
degeneration. 
Natural selection, though consistent with and capable of leading to steady 
_ upward progress and improvement, by no means involves such progress as a 
B necessary consequence. All it says is that those animals will, in each generation, 
have the best chance of survival which are most in harmony with their environ- 
ment, and such animals will not necessarily be those which are ideally the best or: 
most perfect. 
If you go into a shop to purchase an umbrella the one you select is by no 
means necessarily that which most nearly approaches ideal perfection, but the one 
which best hits off the mean between your idea of what an umbrella should be 
and the amount of money you are prepared to give for it: the one, in fact, that is 
on the whole best suited to the circumstances of the case or the environment for 
the time being. It might well happen that you had a violent antipathy to a 
crooked handle, or else were determined to have a catch of a particular kind to 
secure the ribs, and this might lead to the selection, z.¢., the survival, of an 
article that in other and even in more important respects was manifestly inferior to 
the average. 
So is it also with animals: the survival of a form that is ideally inferior 
is very possible. To animals living in profound darkness the possession of eyes 
is of no advantage, and forms devoid of eyes would not merely lose nothing 
_ thereby, but would actually gain, inasmuch as they would escape the dangers that 
_ might arise from injury to a delicate and complicated organ. In extreme cases, 
as in animals leading a parasitic existence, the conditions of life may be such 
as to render locomotor, digestive, sensory, and other organs entirely useless ; 
and in such cases those forms will be best in harmony with their surroundings 
which avoid the waste of energy resulting from the formation and maintenance 
of these organs. 
Animals which have in this way fallen from the high estate of their fore- 
fathers, which have lost organs or systems which their progenitors possessed, are 
1890. 3) ai 
