What is Darwin’s point in chapter 13?
In chapter 13, Darwin presents several ideas in one chapter. The first is that of the classification of life.
“Naturalists try to arrange the species, genera, and families in each class, on what is called the Natural System.”
He then explores some of the criteria in how life are classified.
“almost all naturalists lay the greatest stress on resemblances in organs of high vital or physiological importance”
“Numerous instances could be given of characters derived from parts which must be considered of very trifling physiological importance, but which are universally admitted as highly serviceable in the definition of whole groups.”
“Practically when naturalists are at work, they do not trouble themselves about the physiological value of the characters which they use in defining a group, or in allocating any particular species.”
However, the criteria he presents do not necessarily demonstrate lineage or common ancestry.
The next section briefly discusses morphology. And he states that homologous organs are explainable with natural selection. That might be true, but it is difficult to determine if something is homologous or analagous. And if homologous organs are answered by natural selection, then does analogous organs question it?
Next he discusses embryology.
“It has already been casually remarked that certain organs in the individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike.”
I’m not so convinced that just because embryos look similar that it would show they have a common ancestor.
Finally, he discusses atrophied organs.
“In reflecting on them, every one must be struck with astonishment: for the same reasoning power which tells us plainly that most parts and organs are exquisitely adapted for certain purposes, tells us with equal plainness that these rudimentary or atrophied organs, are imperfect and useless.”
But if rudimentary organs are imperfect and useless, how can they continually pass the filter of natural selection? They should decrease and not gradually improve until it’s a fully developed and functioning organ.
In the case of atrophied organs, natural selection would make sense. The organs would not be useful and would eventually lose value and diminish.