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As a result of the above-mentioned cenogenetic processes those of disturbed and curtailed heredity whole series of lower stages have dropped out in the embryonic development of man and the other mammals especially from the earliest periods, or been falsified by modification. But we find these lower stages in their original purity in the lower vertebrates and their invertebrate ancestors.
We now definitely know that the organic world on our earth has been as continuously developed, "in accordance with eternal iron laws," as Lyell had in 1830 shown to be the case for the inorganic frame of the earth itself; we know that the innumerable varieties of animals and plants which during the course of millions of years have peopled our planet are all simply branches of one single genealogical tree; we know that the human race itself forms only one of the newest, highest, and most perfect offshoots from the race of the Vertebrates.
This is confirmed by a number of interesting facts that have lately come to our knowledge in connection with the embryonic development of the Ceratodus and Lepidosiren; they give us important information as to the stem-history of the lower Vertebrates, and therefore of our early ancestors of the paleozoic age.
The skunk itself offers perhaps the one instance amongst the higher vertebrates of an animal in which all the original instincts of self-preservation have died out, giving place to this lower kind of protection. All the other members of the family it belongs to are cunning, swift of foot, and, when overtaken, fierce-tempered and well able to defend themselves with their powerful well-armed jaws.
Indeed in this respect the worms have been well compared to the relics which fill the shelves of one of our grandmother's china-closets. The four great branches are the echinoderms, mollusks, articulates, and vertebrates. The echinoderms, including starfishes, sea-urchins, and others straggled early from the great army.
To define these powers, we must consider, in the evolution both of the arthropods and the vertebrates, the species which mark the culminating point of each. How is this point to be determined? Here again, to aim at geometrical precision will lead us astray. There is no single simple sign by which we can recognize that one species is more advanced than another on the same line of evolution.
Then the sexes develop their latest function, most prominent among the younger vertebrates, of acting as nature's most potent method of variation and differentiation. In the pursuit of the different, nature has exalted sex, and the intensity of the sex life.
While the Amphioxus is thus connected through the Cyclostoma with the fishes, and so with the series of the higher vertebrates, it is, on the other hand, very closely related to a lowly invertebrate marine animal, from which it seems to be entirely remote at first glance.
The animals, especially the higher vertebrates, of the Pliocene formation on each continent or each larger group of islands, correspond very closely to the now living animals of the same geographical limit, with the exception of being generally of a much larger size.
In most of the higher mammals they pass through this into the scrotum. As a rule, the inguinal canal closes up. The structure of the external sexual organs, the copulative organs that convey the fecundating sperm from the male to the female organism in the act of copulation, is also peculiar to the mammals. There are no organs of this character in most of the other Vertebrates.
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