What is Darwin’s point in chapter 2?
How well does this support his theory of natural selection?
Darwin’s point is that from the species level on down, there is evidence of variation among plants and animals. This points to descent with modification in nature.
“These differences blend into each other in an insensible series; and a series impresses the mind with the idea of an actual passage.”
He mentions several times that it is difficult to define species, sub-species, and varieties. And it is even more difficult to categorize into one of these.
“Nor shall I here discuss the various definitions which have been given of the term species. No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation. The term ‘variety’ is almost equally difficult to define; but here community of descent is almost universally implied, though it can rarely be proved.”
(And I’m not sure what he meant by “act of creation” in the above statement.)
He points out that there is no objective measure of how to classify things.
“Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgement and wide experience seems the only guide to follow. We must, however, in many cases, decide by a majority of naturalists, for few well-marked and well-known varieties can be named which have not been ranked as species by at least some competent judges.”
“From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience sake.”
So, there is a gradual transition between species/subspecies/variaties, and it is only logical that these shared common ancestry. On this point I would not disagree with him.
Where he is lacking in this chapter is evidence for a gradual transition for genera and higher. The only thing he mentions is that a large genus will have a large number of varieties.
“it has invariably proved to be the case that a larger proportion of the species on the side of the larger genera present varieties, than on the side of the smaller genera.”
Though I’m not sure this is any more than a truism.
In this chapter and the previous, the main evidence he presents is changes in the species level and lower. He is not able to demonstrate any hard evidence of a descent with modification on a higher level. But, we’ll wait for further chapters to see if evidence is brought forth.
strevisa wrote:Two points.
1. Species may end up looking like each other by chance.
Darwin’s stuff is based on macroscopic observable traits.
Thus, a dolphin and a shark may have similar fins, but boy are they different!
Yes, Darwin could only go by morphological features. But, I’m not sure what you mean by species could end up looking like each other. Does Darwin mention anything about this in the chapter?
We can’t ask for the impossible: Darwin has no solid explanation for evolution. For a solid explanation we need to wait for experiments around 1940 such as the one with pneumococci inheriting traits shown to be linked to DNA (to the disbelief of the researchers).
I’m not asking for genetic evidence, but simply an extension of his analysis of morphological features.
Here is what I’m noting. Darwin claims that there is evidence of gradual transitioning from species level on down. And I agree with this. But, he does not show that above the species level there is a gradation of morphological features in this chapter. And we’ll have to wait to see if he does present any evidence of this or an explanation to account for why we do not see this.
A general point. As per the guidelines for the book debate, “we are debating the book on its own merits. Usage of modern science to either attack or defend Darwin’s points cannot be used.” So, I’m not asking how modern science addresses the issues, but rather noting how Darwin addresses the issues in the book.
strevisa wrote:Darwin understood that biosystems change and form an array of specimens (similar to a Gaussian curve one can add).
This is probably the best thing he said, and a fact (the Gaussian distribution) that to this day many people fail to grasp.
Not sure what you mean here. Could you quote what Darwin stated in regards to a Gaussian distribution?
Greenbeard wrote:otseng wrote:…he does not show that above the species level there is a gradation of morphological features in this chapter. And we’ll have to wait to see if he does present any evidence of this or an explanation to account for why we do not see this.
I’m not sure what you’re getting at. Why should there be any gradual ‘transition’ between extant species?
What I’m simply trying to point out is that in chapter 2, Darwin presents evidence of species changing, but they do not become a wholly new species. And that we’d have to wait for future chapters to see what evidence he presents that novel species can arise.