Taking Haeckel on his own ground, as above defined, we may next inquire as to the method which he employs in working out his argument. This may be referred to three leading modes of treatment, which, as they are somewhat diverse from those ordinarily familiar to logicians and are extensively used by evolutionists, deserve some illustration, more especially as Haeckel is a master in their use.
An eminent French professor of the art of sleight-of-hand has defined the leading principle of jugglers to be that of "appearing and disappearing things;" and this is the best definition that occurs to me of one method of reasoning largely used by Haeckel, and of which we need to be on our guard when we find him employing, as he does in almost every page, such phrases as "it cannot be doubted," "we may therefore assume," "we may readily suppose," "this afterward assumes or becomes," "we may confidently assert," "this developed directly," and the like, which in his usage are equivalent to the "Presto!" of the conjurer, and which, while we are looking at one structure or animal, enable him to persuade us that it has been suddenly transformed into something else.
In tracing the genealogy of man he constantly employs this kind of sleight-of-hand in the most adroit manner. He is perhaps describing to us the embryo of a fish or an amphibian, and, as we become interested in the curious details, it is suddenly by some clever phrase transformed into a reptile or a bird; and yet, without rubbing our eyes and reflecting on the differences and difficulties which he neglects to state, we can scarcely doubt that it is the same animal, after all.
The little lancelet, or Amphioxus (see Fig. 1), of the European seas—a creature which was at one time thought to be a sea-snail, but is really more akin to fishes—forms his link of connection between our "fish-ancestors" and the invertebrate animals. So important is it in this respect that our author Waxes eloquent in exhorting us to regard it "with special veneration" as representing our "earliest Silurian vertebrate ancestors," as being of "our own flesh and blood," and as better worthy of being an object of "devoutest reverence" than the "worthless rabble of so-called 'saints.'" In describing this animal he takes pains to inform us that it is more different from an ordinary fish than a fish is from a man. Yet, as he illustrates its curious and unique structure, before we are aware, the lancelet is gone and a fish is in its place, and this fish with the potency to become a man in due time. Thus a creature intermediate in some respects between fishes and mollusks, or between fishes and worms, but so far apart from either that it seems but to mark the width of the gap between them, becomes an easy stepping-stone from one to the other.
Fig. 1.
The Lancelet (Amphioxus), the supposed earliest type of vertebrate animal, and, according to Haeckel, the ancestor of man. The figure is a section enlarged to twice the natural size.
a, mouth;
b, anus;
c, gill-opening;
d, gill;
e, stomach;
f, liver;
g, intestine;
h, gill-cavity;
i, notochord, or rudimentary back-bone;
k, l, m, n, o, arteries and veins.
In like manner, the ascidians, or sea-squirts—mollusks of low grade, or, as Haeckel prefers to regard them, allied to worms—are most remote in almost every respect from the vertebrates. But in the young state of some of these creatures, and in the adult condition of one animal referred to this group (Appendicularia), they have a sort of swimming tail, which is stiffened by a rod of cartilage to enable it to perform its function, and which for a time gives them a certain resemblance to the lancelet or to embryo fishes; and this usually temporary contrivance—curious as an imitative adaptation, but of no other significance—becomes, by the art of "appearing and disappearing," a rudimentary backbone, and enables us at once to recognize in the young ascidian an embryo man.
A second method characteristic of the book, and furnishing, indeed, the main basis of its argument, is that of considering analogous processes as identical, without regard to the difference of the conditions under which they may be carried on. The great leading use of this argument is in inducing us to regard the development of the individual animal as the precise equivalent of the series of changes by which the species was developed in the course of geological time. These two kinds of development are distinguished by appropriate names. Ontogenesis is the embryonic development of the individual animal, and is, of course, a short process, depending on the production of a germ by a parent animal or parent pair, and the further growth of this germ in connection more or less with the parent or with provision made by it. This is, of course, a fact open to observation and study, though some of its processes are mysterious and yet involved in doubt and uncertainty. Phylogenesis is the supposed development of a species in the course of geological time and by the intervention of long series of species, each in its time distinct and composed of individuals each going regularly through a genetic circle of its own.