A complete proof of the phylogeny of any creature would be given by the preservation of an unbroken series of all its fossil ancestors. Such a series will in most cases, for obvious reasons, always remain a desideratum. In a few cases, however, the desideratum is nearly met: for example, the ancestral line of the one-toed digitigrade horse from a four-or five-toed plantigrade and still very generalized Ungulate is approaching completion.
Phylogenetic study has to rely upon other help. This is afforded by comparative anatomy and by the study of ontogeny. If the latter were a faithful, unbroken recapitulation of all the stages through which the ancestors have passed, the whole matter would be very simple; but we know for certain that in the individual development many stages are left out (or, rather, are hurried through, and are so condensed by short-cuts being taken that we cannot observe them), while other features which have been introduced obscure, and occasionally modify beyond recognition, the original course.
Again, the sequence of the appearance of the various organs is frequently upset (heterochronism). Some organs are accelerated in their development, while others, which we know to be phylogenetically older, are retarded in making their reappearance in the embryo.
These disturbing or distorting newly introduced features or factors show themselves chiefly in connection with the embryonic conditions of growth—for example, yolk-sac, placenta, amnion. They all come within the category of cænogenesis: they are cænogenetic, while the true, undisturbed recapitulation is palingenetic.
Lastly, some features, so-called rudimentary or vestigial organs, instead of disappearing, are most tenacious in their recurrence, while others of originally fundamental importance scarcely leave recognisable traces, and are, so to speak, only hinted at during the embryonic growth of the creature we happen to study. Hence arises the philosophical study of 'Dysteleology.'
Among other terms invented by Haeckel, and now in general use, are Metamere, Metamerism, Cœlom, Gonochorism, Gastrula, Metazoa, Gnathostomata, Acrania, Craniota, and Amniota.
Hitherto we have dealt with his general work only, a résumé of which he gave for many years in a course of thirty lectures before an audience composed of 'all sorts and conditions of men.' Students of biology and of medicine side by side with theologians, incipient and ordained, jurists, political economists, and philosophers, crowded his lecture-room during the 'seventies to hear the master explaining the 'natural history of creation' or the mysteries of anthropogenesis. Another course of eighty lectures during the winter semester was, and still is, devoted to a systematic treatment of zoology, while practical classes are reserved for the more select.
His winning personality and fascinating eloquence, combined with a clear and concise delivery, have gained the enthusiastic admiration of many a student who went to the quiet University town in order to learn with his own ears and eyes.
List of Separate Publications by Professor Haeckel.