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This shows what great differences may exist in size, in form, and in function, between parts which are developmentally the same, for all these appendages are modifications of one common kind of structure, which becomes differently modified in different situations; in other words, they are serial homologues.
And in this case, the individuals which secreted a fluid, in some degree or manner the most nutritious, so as to partake of the nature of milk, would in the long run have reared a larger number of well-nourished offspring, than would the individuals which secreted a poorer fluid; and thus the cutaneous glands, which are the homologues of the mammary glands, would have been improved or rendered more effective.
Bay Lankester has proposed the term "homoplasy." "Serial homology" is a relation of resemblance existing between two or more parts placed in series one behind the other in the same individual. Examples of such homologues are the ribs, or joints of the backbone of a horse, or the limbs of a centipede. The latter animal is a striking example of serial homology.
Go back to your books." "And as de Roquefort says, 'To sit on a cheese and eat whey is the destiny of fools." "See here, young man," said the beard, ignoring his colleague, "treeness is a life process displaying the aspiration of matter toward hierarchy, order, and structure. It finds analogues and even homologues in life systems everywhere."
In this figure those parts which are identified as homologues of the wings extend wholly across the interior of the food bowl, and have the forms of triangles with smaller triangular spurs at their bases. The wings are extended at right angles to the axis of the body, and taper uniformly to the rim of the bowl.
A cloud came over his rough-hewn majestic visage; he drew himself up, and swayed his body from side to side, and shook his black gown, and lifted his arms, as their plumed homologues are lifted by some great bird, and let them fall again two or three times; and then said, in deep measured tones, which seemed to express rage and despair, "But did you ever see the eagle in his cage?"
The design shown in figure f suggests in many ways a sun-bird, and is comparable with those previously studied and illustrated. There is no question of the homologues of tail, head, and wings. The meridional band across the bowl is similar to those already discussed, and its relationship to the head and tail of the bird identical.
H. Spencer seems to deny serial homology to the mollusca, but it is difficult to see why the shell segments of chiton are not such homologues because the segmentation is superficial. Similarly the external processes of eolis, doris, &c., are good examples of serial homology, as also are plainly the successive chambers of the orthoceratidæ.
Serial homologues so formed might be called, as Mr. Ray Lankester has proposed, "homoplastic." But there are, it is here contended, abundant reasons for thinking that the predominant agent in the production of the homologies of the limbs is an internal force or tendency.
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