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Classification, groups subordinate to groups Natural system Rules and difficulties in classification, explained on the theory of descent with modification Classification of varieties Descent always used in classification Analogical or adaptive characters Affinities, general, complex and radiating Extinction separates and defines groups Morphology, between members of the same class, between parts of the same individual Embryology, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age Rudimentary organs; their origin explained Summary.
Boyle, unlike Browne, exposed embryos of different ages to the action of "Spirit of Wine" or "Sal Armoniack," demonstrating thereby the chemical fixation of embryos as an aid to embryology.
If Meckel's proposition is so far qualified, that the comparison of adult with embryonic forms is restricted within the limits of one type of organisation; and, if it is further recollected that the resemblance between the permanent lower form and the embryonic stage of a higher form is not special but general, it is in entire accordance with modern embryology; although there is no branch of biology which has grown so largely, and improved its methods so much, since Meckel's time, as this.
If we appeal from adult anatomy to embryology the case becomes all the worse for us. Our ear is lodged in the gill-slit of a fish, our jaws are branchial arches, our hyoid bone the rudiment of this system of bones supporting the gills.
In our discussion on embryology, we shall see why such characters are so valuable, on the view of classification tacitly including the idea of descent. Our classifications are often plainly influenced by chains of affinities. Nothing can be easier than to define a number of characters common to all birds; but in the case of crustaceans, such definition has hitherto been found impossible.
All the larvæ of insects are therefore like one another, and like worms also, in certain fundamental characters of internal and external structure; so the conclusion that the whole group of insects has arisen by evolution from more primitive ancestors resembling the worms of to-day is based upon mutually explanatory details of comparative anatomy and embryology.
But embryology answers by showing us the highest and most complex forms of life attained every day from very elementary forms; and palaeontology, as it develops, allows us to witness the same spectacle in the universal history of life, as if the succession of phases through which the embryo passes were only a recollection and an epitome of the complete past whence it has come.
All the peculiarities that accompany and follow the formation of the amnion, and that we have learned in our consideration of human embryology; all the peculiarities in the development of the organs which we will presently follow in detail; finally, all the principal special features of the internal structure of the full-grown Amniotes prove so clearly the common origin of all the Amniotes from single extinct stem-form that it is difficult to entertain the idea of their evolution from several independent stems.
So long, therefore, as our morphological knowledge was almost wholly confined to anatomy, the characters of groups were solely anatomical; but as the phenomena of embryology were explored, the likeness and unlikeness of individual development had to be taken into account; and, at present, the study of ancestral evolution introduces a new element of likeness and unlikeness which is not only eminently deserving of recognition, but must ultimately predominate over all others.
Its three chapters treat successively of this order of Reptiles, of its families, of its North American genera and species. The THIRD PART, contained in the second volume, is entitled, "Embryology of the Turtle." It consists of two chapters: "Development of the Egg, from its first appearance to the formation of the embryo."
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