in so much greater as the Invertebrata, with perhaps the exception of the Articulata and the Radiata, are less pronouncedly typical and physiologically less intelligible.
This uncertainty of the laws of organic structure comes from the fact that these laws themselves are purely experimental, containing in themselves no ultimate, logically cogent truth such as we find in physico-mathematical laws. Hence, a departure of nature from these rules implies no contradiction, no impossibility; and if a thing is not impossible, of course it is possible.
Physico-mathematical laws form, as it were, a resting-place on which we may safely step without fearing that it will ever fail under our weight. In the history of development, on the other hand, what has been our experience? Within a short period of time, a very restricted survey of the animal world, a survey guided by chance, made us acquainted with a series of facts that conflict with all our previous knowledge. Discoveries like that of the inverted position of the embryo in certain rodents, of the development in the deer, of alternate generations, of the development of the echinoderms, of the Entoconcha mirabilis, of parthenogenesis, of hectocotyly—all these are calculated to put us on our guard against premature generalization in this field. But in fact such anomalies as these are only counterparts of others with which we have long been familiar, as the marsupials, viviparous fishes, etc., which make no impression on us, because already known to science.
Under such circumstances the application of the biogenetic fundamental law to individual cases is very hazardous, even though we admit the principle in a general sense. The inferences which ontogeny, guided by a few scattered paleontological characters, permits us to draw with regard to phylogeny, will never possess more than a very restricted degree of probability. It will ever be open to the individual understanding to take whatever way it chooses amid the confusion of innumerable and complex possibilities, and, excepting a few indisputable points, which, however, were understood long ago, to conceive a great many different modes of development of the organic world as it exists now. As for certain genealogies of our race drawn up in unfettered presumptuousness rather by an artistic imagination than by a scientifically trained mind, they are of about the same value as the pedigrees of Homeric heroes in the eyes of the historical critic. For my part, if I want to read a work of fiction, I can find something better than a "History of Creation."
But this is not the point which concerns us just now. Granting the scheme of descent from the little mass of protoplasm with which life is supposed to have begun, up to man himself, to be clearly made out (which it is not), the fashioning of organic nature will, after all, be as great a riddle as ever, if laws of structure have alone determined its development.
And this, not because molecular mechanics, which produces modifi-
