Variation...again? The answers for the question we can't answer. (Chapter 5)

Chapter 5 "Laws of Variation" begins,

"I have hitherto sometimes spoken as if the variations...had been due to chance. This, of course, is a wholly incorrect expression" (131).

1. Yes, Darwin is discussing variations again.

2. Darwin gives possible explanations for why variations occur, and then goes on to say, "Not in one case out of a hundred can we pretend to assign any reason why this or that part differs" (167).

But, never mind that. Even if we cannot decide why each organism has varied the way it has, it comes in handy to know the possible causes of the variation. So, now that we have a working theory of Natural Selection, we are returning to the 'raw material' of variations and exploring its causes.

As for the content of the chapter, Darwin is pretty straight forward in this one. So I will point out the headings of the sections and explain what he means by them (except the two headings that are self-explanatory, as you will see):

--(beginning of the chapter - no heading)

The changes or disturbances in the parents' reproductive systems is the main cause for change in the offspring.

--Effects of Use and Disuse

Use of an attribute of an organism strengthens that attribute, and disuse of an attribute diminishes it. If an organism has no need for an organ or a limb, it may, over time, vary to have a diminished version of that organ or limb (or none at all).

--Acclimatisation

"The degree of adaptation of species to the climates under which they live is often overrated" (139). We often think of parts of a species being attributed to its climate, ie. polar bears' fur. But actually, the climate is only part of the equation, and polar bears have their specific kind of fur for many reasons that we cannot pinpoint.

--Correlation of Growth

Sometimes when one variation occurs, another occurs in the same animal that correlates to it. Especially as an embryo, where certain parts of the body are homologous, those parts might vary together.

--"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" (150).

Yes, that is a title. It means that if a species develops a part abnormally (compared to other similar species), that part is extremely variable. This would not apply to the wings of bats developing, for example, because other bats have wings as well (150).

--"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" (159).

Different species can vary in similar ways and can also revert back to characteristics of an ancestor.

The rest is examples and evidence. Important stuff, I know, but I could not possibly rewrite the whole book here. So, if you are interested in that part, check out Chapter 5 of The Origin.

Darwin ends the chapter on a strong note once again, driving home the fact that all of this does relate to and support his theory:

"...it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enables to struggle with each other, and the best adapted to survive" (170).

Tune in next time for...The Difficulties! (cue ominous music)