Connecting Links.
If one group of animals has been derived by descent from another there should be some form more or less intermediate between the two and with some characteristics of both groups. Many such connecting links actually exist at the present time. Almost every order of animals possesses some primitive members that have, doubtless, evolved at a slower rate than their relatives and lave on that account retained a larger measure of ancestral traits than have the more typical representatives of the group. Thus there is a group of primitive annelid worms, represented by Dinophilns, Protordrillus and Pollygordius, that serve partially to bridge the gap between the two grand divisions, annolide and flatworms. The case of the several species of Dinophilus is especially noteworthy, for these little animals are so evenly balanced between the characteristics of one phylum and those of the other that some authors place them among the flatworms, others among the annelids and still others are inclined to place them in an anomalous group by themselves. There is an interesting genus of primitive centipes, called Pertpatus, which possess as many annelid features as anthropoid features. Among vertebrates we have the familiar example of the lung fishes with both the gills of fishes and lungs homologous with those of land vertebrates. And finally, we may mention those curious egg-laying mammals, momolrems, of Australia and New Zealand, which though obviously mamalian in most respects, possess, in addition of laying eggs after the fashion of reptiles, many other decidedly reptilian traits. The reader interested in following up in more detail this interesting branch of comparative anatomy will find the subject skillfully handled by Geoffrey Smith in a volume entitled "Primitive Animals."
Comparative anatomy is a mature and well organzed science and involves a vast amount of technical data. No one but a trained comparative anatomist can reasonably be expected to appreciate the dependence of this subject upon the principle of evolution. Without evolution as a guiding principle, comparative anatomy would be a hopeless mass of meaningless and disconnected facts; with the aid of the principle of homology, an evolutionary assumption, it has grown to be one of the most scientific branches of biology. This may be faken as an illustration of the nature of the proof of organic evolution; that when it is used as a working hypothesis or guiding principle it really works in that it is not only consistent with all of the facts, but lends significance and interest to facts that would otherwise be drab and disconnected.
Evidences From Classification.
The object of classification is to arrange all species of animals and plants in groups of various degrees of inclusiveness which shall express as closely as possible the actual degrees of relationship existing between them. In pursuance of this object we begin by grouping together as one species all animals that are essentially alike in their anatomical details. As an example of the methods of classification we may take the following familiar instance: The European wolf is a particular kind of animal constituting a species called lupus (the Latin word for wolf), all members of which are more like one another than they are like wolves of other sorts, for the reason that they have a common inherilance. There are not a few other species of wolves, each given a Latin name, and all of these wolf species, including dogs (believed to be domesticated and, therefore, highly modified wolves), are placed in one genus, Canis. Several other genera of more or less wolflike animals, such as jackals and foxes, are grouped with the genus Canis, and constitute the family Canidae, the assumption being that they are all the diversified descendants of some common wolflike ancestor. Other families, such as the cat family (felidae), the bear family (urisdae) and several other
