eg 
432 
usually adhered to their support in clusters, the case of the genus 
Acasta occurred to me, in which all the species are embedded in 
sponges, generally at some little distance from each other ; I then 
turned to my description of the animal, and found it stated, that 
in several of the species the probosci-formed penis is ‘‘ remarkably 
long ;” and this I think can hardly be an accidental coincidence. 
With respect to the habits of the genera which are provided 
with true males or complemental males :—all the species of 
Scalpellum, excepting one, are specially modified for attachment 
to the delicate branches of corallines: the one species of Ibla, 
about which I know anything, lives attached, generally two or 
three together, to the peduncle of another cirripede, viz. a 
Pollicipes: Alcippe and Cryptophialus are embedded in small 
cavities which they excavate in shells. No douwt in all these 
cases two or more full-grown individuals might become attached 
close together to the same support ; and this sometimes occurs 
with Scalpellum vulgare, but the individuals in such groups are 
apt to be distorted and to have their peduncles twisted. There 
would be much difficulty ia two or more individuals of Alcippe 
and Cryptophialus living embedded in the same cavity. More- 
over, it might well happen that sufficient food would not be 
brought by the currents of the sea to several individuals of these 
species living close together. Nevertheless in all these cases it 
would be a manifest advantage to the species, if two individuals 
could live and flourish close together, so as occasionally to inter- 
cross. Now if certain individuals were reduced in size and 
transmitted this character, they could readily be attached to the 
other and larger individuals ; and as the process of reduction 
was continued, the smaller individuals would be enabled to adhere 
closer and closer to the orifice of the sack, or, as actually occurs 
with some species of Scalpellum and with Ibla, within the sack of 
the larger individual ; and thus the act of fertilisation would be 
safely effected. It is generally admitted that a division of phy- 
siological labour is an advantage to all organisms ; accordingly, 
a separation of the sexes would be so to cirripedes, that is if this 
could be effected with full security for the propagation of the 
species. How in any case a tendency to a separation of the 
sexes first arises, we do not know; but we can plainly see that 
if it occurred in the present case, the smaller individuals would 
almost necessarily become males, as there would be much less 
expenditure of organic matter in the production of the spermatic 
fluid than of ova. Indeed with Scalpellum vulgare the whole 
body of the male is smaller than a single one of the many ova 
produced by the hermaphrodite. The other and larger indi- 
viduals would on the same principle either remain hermaphro- 
dites, but with their masculine organs more or less reduced, or 
would be converted into females. At any rate, whether these views 
are correct or not, we see at the present time within the genus 
Scalpellum a graduated series : first on the masculine side, from an 
animal which is obviously a pedunculated cirripede with well- 
proportioned valves, to a mere sack enclosing the male organs, 
either with the merest rudiments of valves, or entirely destitute 
of them ; and secondly on the feminine side, we have either 
true females, or hermaphrodites with the male organs perfect, 
yet greatly reduced. 
With respect to the means by which so many of the most im- 
portant organs in numerous animals and plants have been greatly 
reduced in size and rendered rudimentary, or have been quite 
obliterated, we may attribute much to the inherited effects of the 
disuse of parts. But this would not apply to certain parts, for 
instance to the calcareous valves of male cirripedes which cannot 
be said to be actively used. Before I read Mr. Mivart’s acute 
criticisms on this subject, I thought that the principle of the 
economy of growth would account for the continued reduction 
and final obliteration of parts; and I still think, that during the 
earlier periods of reduction the process would be thus greatly 
aided. But if we consider, for instance, the rudimentary pistils 
SR naa ef Oa 
NATURE 
[Sepz. 25, 1873 
a aa cee 
or stamens of many plants, it seems incredible that the reduc- 
tion and final obliteration of a minute papilla, formed of mere 
cellular tissue, could be of any service to the species. The fol- 
lowing conjectural remarks are made solely in the hope of calling 
the attention of naturalists to this subject. It is known from 
the researches of Quetelet on the height of man, that the num- 
ber of individuals who exceed the” average height by a given 
quantity is the same as the number of those who are shorter than 
the average by the same quantity ; so that men may be grouped 
symmetrically about the average with reference to their height. 
I may add, to make this clearer, that there exists the same 
number of men between three and four inches above the average 
height, as there are below it. So it is with the circumference 
of their chests ; and we may presume that this is the usual law 
of variation in all the parts of every species under ordinary 
conditions of Jife. That almost every part of the body is 
capable of independent variation we have good reason to believe, 
for it is this which gives rise to the individual differences charac- 
teristic of all species. Now it does not seem improbable that 
with a species under unfavourable conditions, when, during many 
generations, orjin certain areas, it is pressed for food and exists in 
scanty numbers, that all or most of its parts should tend to vary in 
a greater number of individuals towards diminution than towards 
increment of size; so that the grouping would be no longer 
symmetrical with reference to the average size of any organ under 
consideration. In this case the individuals which were born 
with parts diminished in size and efficiency, on which the welfare 
of the species depended, would be eliminated ; those individuals 
alone surviving in the long run which possessed such parts of 
the proper size. But the survival of none would be affected by 
the greater or less diminution of parts already reduced in size 
and functionally useless. We have assumed that under the 
above stated unfavourable conditions alarger number of indivi- 
duals are born with any particular part or organ diminished in 
size, than are born with it increased to the same relative degree ; 
and as these individuals, having their already reduced and useless 
parts still more diminished by variation under poor conditions, 
would not be eliminated, they would intercross with the 
many individuals having the part of nearly average size, and 
with the few having it of increased size. The result of such 
intercrossing would be, in the course of time, the steady diminu- 
tion and ultimate disappearance of all such useless parts. No 
doubt the process would take place with excessive slowness ; 
but this result agrees perfectly with what we see in nature ; 
for the number of forms possessing the merest traces of various 
organsis immense. I repeat that I have ventured to make these 
hypothetical remarks solely for the sake of calling attention to 
this subject. CHARLES DARWIN 
Down, Beckenham, Kent, Sept. 20 
Reflection of the Rainbow 
Draw a circle to represent a rain-drop, or rather a section of 
it, by a plane passing through its centre, the sun, and the eye. 
Draw a straight line through the centre to represent a solar ray 
of mean refrangibility. At the front and back of the drop re- 
flection occurs, and the incidence being normal, the incident and 
reflected beams will coincide after the emergence of the latter 
from the drop. Now suppose the ray through the centre to move 
parallel to itself, the incidence grows more and, more oblique, 
refraction occurs at entrance and at emergence, the ray finally 
becoming a tangent to the drop. Let the incident and the twice 
refracted and once reflected rays be produced backwards till 
they intersect behind the drop : the angle enclosed between them 
augments with the obliquity, reaches a maximum, and then 
diminishes. The ray corresponding in obliquity with this maxi- 
mum angular value, and those in its immediate vicinity, quit the 
drop sensibly parallel, and these are the rays which are effectual | 
in the rainbow. .This angle being for red light 42°, and for 
violet light 40°, for light of mean refrangibility it is 41°. 
a If those parallel rays before reaching the observer's eye impinge 
