Morphological Resemblance or Homology

Nevertheless they are regarded as homologous, even although no real intermediates are yet known.

This belief is founded largely on the identity of the nerve supply : it having been found that the evidence as to homology afforded by different classes of structures varies greatly in value.

But it is possible to carry the conception of homology much further ; an investigation of the origin of the nerves supplying the fore limb shows that that structure, which is clearly homologous among the vertebrates, may lie at very varying distances behind the head ; in a frog its supply comes from the actual second, third and fourth spinal nerves behind the head; in the extinct reptile "Elasmosaurus" it probably came from the 77th to the Both nerves.

Thus different position in the body of an animal, whose body is in part or whole composed of a succession of similar segments is consistent with homology. But in general character, in mode of origin, and in the nature of their nerve supply, the fore and hind limbs of vertebrates are identical, and thus by an extension of reasoning may be held to be homologous, the homology being of a special nature called serial.

The criteria which may be used for the determination of serial homology between the parts of an individual are in essence identi cal with those used in discussions involving two distinct animals, identity of general relationships and embryological origin being those chiefly used.

Analysis of the Vertebrate

the most elaborate example of an analysis of structure depending on the determination of serial homology is that of the vertebrate head, first attempted successfully by J. W. van Wijhe. A long series of investigations has led morphologists to the view that the head of a vertebrate includes a series of segments, eight in number in most higher forms, which are homologous with one another and with the segments of the trunk. From this conception the pre diction could be made that there must once have existed primitive vertebrates in which each segment contained a definite series of muscles, skeletal parts, nerves, blood vessels and gills either identical throughout the series, or exhibiting a gradation in struc ture from front to back. Such an animal has now been discovered in the fossil "lamprey" Cephalaspis, from the Upper Silurian. Such verification of a prediction by subsequent discovery is the best testimony to the value of the method by which the original prediction was made.