Chapter 5 – Law of Variation

What is the point of chapter 5?

This chapter discusses some of the (nonrandom) causes of variations.

“I have hitherto sometimes spoken as if the variations so common and multiform in organic beings under domestication, and in a lesser degree in those in a state of nature had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation. Some authors believe it to be as much the function of the reproductive system to produce individual differences, or very slight deviations of structure, as to make the child like its parents. But the much greater variability, as well as the greater frequency of monstrosities, under domestication or cultivation, than under nature, leads me to believe that deviations of structure are in some way due to the nature of the conditions of life, to which the parents and their more remote ancestors have been exposed during several generations.”

From my understanding, this is different from the modern day understanding of evolution, in which practically all modifications are a result of random mutations. In addition to chance modifications, Darwin posits that there are external influences that directly affect changes in organisms, though he’s not sure how much of an effect they have.

“How much direct effect difference of climate, food, &c., produces on any being is extremely doubtful. My impression is, that the effect is extremely small in the case of animals, but perhaps rather more in that of plants.”

“When a variation is of the slightest use to a being, we cannot tell how much of it to attribute to the accumulative action of natural selection, and how much to the conditions of life.”

The first example he gives is “use and disuse” (though his examples are all disuse rather than use) and looks into flightless birds and blind animals.

Next, he talks about acclimatisation.

“Hence I am inclined to look at adaptation to any special climate as a quality readily grafted on an innate wide flexibility of constitution, which is common to most animals. On this view, the capacity of enduring the most different climates by man himself and by his domestic animals, and such facts as that former species of the elephant and rhinoceros were capable of enduring a glacial climate, whereas the living species are now all tropical or sub-tropical in their habits, ought not to be looked at as anomalies, but merely as examples of a very common flexibility of constitution, brought, under peculiar circumstances, into play.”

In this chapter, he tries to give a definition of “Correlation of Growth”.

“I mean by this expression that the whole organisation is so tied together during its growth and development, that when slight variations in any one part occur, and are accumulated through natural selection, other parts become modified. This is a very important subject, most imperfectly understood.”

He notes that characteristics of animals/plants are sometimes found in pairs. Blue eye and deafness in cats. Outer and inner flowers in some plants.

And he also mentions a related idea the “law of compensation or balancement of growth”.

“In order to spend on one side, nature is forced to economise on the other side”

At the end, he admits that we cannot know for certainty the causes of variations.

“Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part differs, more or less, from the same part in the parents.”

In the chapter, he mentions the theory of creation many times. And as before, the view of creation that he attacks is not necessarily the Biblical version of creation.

“According to the ordinary view of each species having been independently created, we should have to attribute this similarity in the enlarged stems of these three plants, not to the vera causa of community of descent, and a consequent tendency to vary in a like manner, but to three separate yet closely related acts of creation.”

I’m not sure what he means by “separate yet closely related acts of creation”. Further, the Bible mentions that “kinds” of animals/plants were created. It doesn’t necessarily mean that kinds are species. Further, kinds of animals went on Noah’s ark. And they didn’t have to be species either.

In this chapter, the arguments that he gives do not really strengthen his theory. Examples that he gives of change are from birds that fly to birds that do not fly. But, that seems simpler to explain than how birds got to have wings in the first place. And same with having eyes that are blind, than the origin of eyes.

Finally, another weakness of the chapter is that his statements can be interpreted as espousing Lamarckism.

“leads me to believe that deviations of structure are in some way due to the nature of the conditions of life, to which the parents and their more remote ancestors have been exposed during several generations.”