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05 - Laws of Variation 05-08 - Parts Developed in an Unusual Manner are Highly Variable 20 Several years ago I was much struck by a remark, to the above effect, made by Mr. Waterhouse.
George Robert Waterhouse
George Robert Waterhouse


Professor Owen, also, seems to have come to a nearly similar conclusion.
Richard Owen
Richard Owen


It is hopeless to attempt to convince any one of the truth of the above proposition without giving the long array of facts which I have collected, and which cannot possibly be here introduced.

I can only state my conviction that it is a rule of high generality. I am aware of several causes of error, but I hope that I have made due allowance for them. It should be understood that the rule by no means applies to any part, however unusually developed, unless it be unusually developed in one species or in a few species in comparison with the same part in many closely allied species.

Thus, the wing of a bat is a most abnormal structure in the class of mammals, but the rule would not apply here, because the whole group of bats possesses wings; it would apply only if some one species had wings developed in a remarkable manner in comparison with the other species of the same genus.

bat
bat


The rule applies very strongly in the case of secondary sexual characters, when displayed in any unusual manner.

The term, secondary sexual characters, used by Hunter, relates to characters which are attached to one sex, but are not directly connected with the act of reproduction.

The rule applies to males and females; but more rarely to the females, as they seldom offer remarkable secondary sexual characters.

The rule being so plainly applicable in the case of secondary sexual characters, may be due to the great variability of these characters, whether or not displayed in any unusual manner- of which fact I think there can be little doubt.

But that our rule is not confined to secondary sexual characters is clearly shown in the case of hermaphrodite cirripedes; I particularly attended to Mr. Waterhouse's remark, whilst investigating this Order, and I am fully convinced that the rule almost always holds good.
cirripede
cirripede


I shall, in a future work, give a list of all the more remarkable cases; I will here give only one, as it illustrates the rule in its largest application.

The opereular valves of sessile cirripedes (rock barnacles) are, in every sense of the word, very important structures, and they differ extremely little even in distinct genera; but in the several species of one genus, Pyrgoma, these valves present a marvelous amount of diversification; the homologous valves in the different species being sometimes wholly unlike in shape; and the amount of variation in the individuals of the same species is so great, that it is no exaggeration to state that the varieties of the same species differ more from each other in the characters derived from these important organs, than do the species belonging to other distinct genera.
05 - Laws of Variation 05-07 - Multiple, Rudimentary, and Lowly-organised Structures are Variable 20 I presume that lowness here means that the several parts of the organisation have been but little specialised for particular functions; and as long as the same part has to perform diversified work, we can perhaps see why it should remain variable, that is, why natural selection should not have preserved or rejected each little deviation of form as carefully as when the part has to serve for some one special purpose.

In the same way, a knife which has to cut all sorts of things may be of almost any shape; whilst a tool for some particular-purpose must be of some particular shape.

knife
knife


Natural selection, it should never be forgotten, can act solely through and for the advantage of each being. Rudimentary parts, as it is generally admitted, are apt to be highly variable.

We shall have to recur to this subject; and I will here only add that their variability seems to result from their uselessness, and consequently from natural selection having had no power to check deviations in their structure.
05 - Laws of Variation 05-09 - Specific Characters more Variable than Generic Characters 20 On the ordinary view of each species having been independently created, why should that part of the structure, which differs from the same part in other independently created species of the same genus, be more variable than those parts which are closely alike in the several species?

I do not see that any explanation can be given.

But on the view that species are only strongly marked and fixed varieties, we might expect often to find them still continuing to vary in those parts of their structure which have varied within a moderately recent period, and which have thus come to differ.

Or to state the case in another manner:- the points in which all the species of a genus resemble each other, and in which they differ from allied genera, are called generic characters; and these characters may be attributed to inheritance from a common progenitor, for it can rarely have happened that natural selection will have modified several distinct species, fitted to more or less widely-different habits, in exactly the same manner: and as these so-called generic characters have been inherited from before the period when the several species first branched off from their common progenitor, and subsequently have not varied or come to differ in any degree, or only in a slight degree, it is not probable that they should vary at the present day.

On the other hand, the points in which species differ from other species of the same genus are called specific characters; and as these specific characters have varied and come to differ since the period when the species branched off from a common progenitor, it is probable that they should still often be in some degree variable,- at least more variable than those parts of the organisation which have for a very long period remained constant.
05 - Laws of Variation 05-10 - Secondary Sexual Characters Variable 20 It is a remarkable fact, that the secondary differences between the two sexes of the same species are generally displayed in the very same parts of the organisation in which the species of the same genus differ from each other.

Of this fact I will give in illustration the two first instances which happen to stand on my list; and as the differences inthese cases are of a very unusual nature, the relation can hardly be accidental.

The same number of joints in the tarsi is a character common to very large groups of beetles, but in the Engidoe, as Westwood has remarked, the number vary greatly; and the number likewise differs in the two sexes of the same species.

beetle
beetle


Again in the fossorial hymenoptera, the neuration of the wings is a character of the highest importance, because common to large groups; but in certain genera the neuration differs in the different species, and likewise in the two sexes of the same species.

Sir J. Lubbock has recently remarked, that several minute crustaceans offer excellent illustrations of this law.

"In Pontella, for instance, the sexual characters are afforded mainly by the anterior antennae and by the fifth pair of legs: the specific differences also are principally given by these organs."

crustacean
crustacean

Pontella
Pontella


This relation has a clear meaning on my view: I look at all the species of the same genus as having as certainly descended from a common progenitor, as have the two sexes of any one species.

Consequently, whatever part of the structure of the common progenitor, or of its early descendants, became variable, variations of this part would, it is highly probable, be taken advantage of by natural and sexual selection, in order to fit the several species to their several places in the economy of nature, and likewise to fit the two sexes of the same species to each other, or to fit the males to struggle with other males for the possession of the females.