Development a Recapitulation of Ancestral History
If all livng beings have come into existence by the gradual modification, through a long series of generations, of a primordial living matter, the phenomena of embryonic development ought to be explicable as particular cases of the general law of hereditary transmission. On this view, a tadpole is first a fish, and then a tailed amphibian, provided with both gills and lungs, before it becomes a frog, because the frog was the last term in a series of modifications whereby some ancient fish became an urodele amphibian; and the urodele amphibian became an anurous amphibian. In fact, the development of the embryo is a recapitulation of the ancestral history of the species.
If this be so, it follows that the development of any organism should furnish the key to its ancestral history; and the attempt to decipher the full pedigree of organisms from so much of the family history as is recorded in their development has given rise to a special branch of biological speculation, termed phylogeny.
In practice, however, the reconstruction of the pedigree of a group from the development history of its existing members is fraught with difficulties. It is highly probable that the series of developmental stages of the individual organism never presents more than an abbreviated and condensed summary of ancestral conditions; while this summary is often strangely modified by variation and adaptation to conditions; and it must be confessed that, in most cases, we can do little better than guess what is genuine recapitulation of ancestral forms, and what is the effect of comparatively late adaptation.
Palaeontology and Evolution
The only perfectly safe foundation for the doctrine of Evolution lies in the historical, or rather archaeological, evidence that particular organisms have arisen by the gradual modification of their predecessors, which is furnished by fossil remains. That evidence is daily increasing in amount and in weight; and it is to be hoped that the comparison of the actual pedigree of these organisms with the phenomena of their development may furnish some criterion by which the validity of phylogenetic conclusions, deduced from the facts of embryology alone, may be satisfactorily tested.
Haeckel, Generelle Morphologie; H. Spencer, Principles of Biology.
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Biology - Table of Contents
The above article was written by Thomas Henry Huxley, M.D., LL.D., F.R.S., President of the Royal Society, 1883-85; Professor of Natural History in the Royal School of Mines, London, 1854; author of Theory of the Vertebrate Skull, The Physical Basis of Life, Introduction to the Classification of Animals, Lay Sermons, Elementary Physiology, etc.