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But among the mammals this cloaca is only permanent in the Monotremes, as it is in all the birds, reptiles, and amphibia. The use that we have hitherto made of our biogenetic law will give the reader an idea how far we may trust its guidance in phylogenetic investigation.
But what has all this to do with descent? Even the “biogenetic law” itself, especially if it were correct, would fit admirably into the frame of the pure evolution idea. Indeed, the older doctrine of evolution took account of this long ago.
This remarkable metamorphosis of the Amphibia is very instructive in connection with our human genealogy, and is particularly interesting from the fact that the various groups of actual Amphibia have remained at different stages of their stem-history, in harmony with the biogenetic law.
Throughout the volume one sees the adoption of the broad biological standpoint in mental life. The adoption of the term "biogenetic psychoses" is indicative of the general trend. The adoption of this well-chosen phrase is, I venture to suggest, the product of Dr. Meyer. The reviewer regrets that the papers do not very well lend themselves for brief reviews.
Thorough opponents of the biogenetic law have laid great stress on these and similar cenogenetic phenomena, and endeavoured to urge them as striking disproofs of the law. As in every other instance, careful, discriminating, comparative-morphological examination converts these supposed disproofs of evolution into strong arguments in its favour.
In entering the obscure paths of this phylogenetic labyrinth, clinging to the Ariadne-thread of the biogenetic law and guided by the light of comparative anatomy, we will first, in accordance with the methods we have adopted, discover and arrange those fragments from the manifold embryonic developments of very different animals from which the stem-history of man can be composed.
Such a problem, however, as the one contained in that biogenetic maxim, which only gives to investigators the direction in which possibly an interesting and profitable path can be opened, does not at all deserve the name of a "law."
But without the biogenetic law, without the distinction between palingenesis and cenogenesis, and without the theory of evolution on which we base it, it is quite impossible to understand the facts of organic development; without them we cannot cast the faintest gleam of explanation over this marvellous field of phenomena.
This important causal nexus finds its simplest expression in "the fundamental law of organic development," the content and purport of which we have fully considered in the first chapter. According to this biogenetic law, ontogeny is a brief and condensed recapitulation of phylogeny.
The more perfect the animal is, the longer is the series of forms it passes through.” So J. Fr. Meckel wrote in 1812 in his “Handbook of Pathological Anatomy,” with no thought of descent. And the facts which led to the construction of the biogenetic law were discovered in no small measure by Agassiz, who was an opponent of the doctrine of descent.
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