16. Laws of Variation III - More laws

Continuing with the bite-sized review of On the Origin of Species...

More Laws of Variation

i. Compensation and Balancement of Growth

Unused parts dwindle because there is a energy cost:

"I suspect, also, that some of the cases of compensation which have been advanced, and likewise some other facts, may be merged under a more general principle, namely, that natural selection is continually trying to economise in every part of the organisation. If under changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure. I can thus only understand a fact with which I was much struck when examining cirripedes, and of which many other instances could be given: namely, that when a cirripede is parasitic within another and is thus protected, it loses more or less completely its own shell or carapace."

ii. Multiple and rudimentary parts are subject to more variation

It had been noticed by botanists that, for instance, that flowers who have a few parts repeated (i.e stamens or petals) often do so in a fixed quantity, say 5. However, other flowers, with multiple parts repeated, will have a varying quantity of those parts.

Darwin on rudimentary (vestigial) organs:

"Rudimentary parts, it has been stated by some authors, and I believe with truth, are apt to be highly variable. We shall have to recur to the general subject of rudimentary and aborted organs; and I will here only add that their variability seems to be owing to their uselessness, and therefore to natural selection having no power to check deviations in their structure. Thus rudimentary parts are left to the free play of the various laws of growth, to the effects of long-continued disuse, and to the tendency to reversion."

iii. A part developed in any species in an extraordinary degree or manner, in comparison with the same part in allied species, tends to be highly variable.

Related species will show the most variation in the same parts. To explain this, Darwin cites again from his own observations with barnacles:

"I am fully convinced that the rule almost invariably holds good with cirripedes. I shall, in my future work, give a list of the more remarkable cases; I will here only briefly give one, as it illustrates the rule in its largest application. The opercular valves of sessile cirripedes (rock barnacles) are, in every sense of the word, very important structures, and they differ extremely little even in different genera; but in the several species of one genus, Pyrgoma, these valves present a marvellous 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 several of the species is so great, that it is no exaggeration to state that the varieties differ more from each other in the characters of these important valves than do other species of distinct genera."

An example of variation in the horns of Western Hercules Beetles, Dynastes granti can be found on Alex Wild's site.

iv. Distinct species present analogous variations; and a variety of one species often assumes some of the characters of an allied species, or reverts to some of the characters of an early progenitor.

For an example of this see the previous blog entry, Darwin Speaks - Why stripes on horses?

Next, Difficulties on Theory - in which Darwin anticipates arguments against Natural Selection.