At the present time it is no longer a debatable question whether or no Evolution has taken place. Since the time of Darwin the accumulation of facts in its support has been so overwhelming that all zoologists look upon this question as settled, and desire now to find out the manner in which such evolution has taken place. Here two problems offer themselves for investigation, which can be and are treated separately—the one dealing with the question of those laws of heredity and variation which have brought about in the past and are still causing in the present the evolution of living beings, i.e. the causes of evolution; the other concerned with the relationship of animals, or groups of animals, rather than with the causes which have brought about such relationship, i.e. the sequence of evolution.
It is the latter problem with which this book deals, and, indeed, not with the whole question at all, but only with that part of it which concerns the origin of vertebrates.
This problem of the sequence of evolution is of a twofold character: first, the finding out of the steps by which the higher forms in any one group of animals have been evolved from the lower; and secondly, the evolution of the group itself from a lower group.
In any classification of the animal kingdom, it is clear that large groups of animals exist which have so many common characteristics as to necessitate their being placed in one larger group or kingdom; thus zoologists are able to speak definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Cœlenterata, Mollusca, etc. In each of these groups affinities can be traced between the members, so that it is possible to speak of the progress from lower to higher members of the group, and it is conceivable, given time to work out the details, that the natural relationships between the members of the whole group will ultimately be discovered.
Thus no one can doubt that a sequence of the kind has taken place in the Vertebrata as we trace the progress from the lowest fishes to man, and already the discoveries of palæontology and anatomy give us a distinct clue to the sequence from fish to amphibian, from amphibian to reptile, from reptile to mammal on the one hand, and to bird on the other. That the different members of the vertebrate group are related to each other in orderly sequence is no longer a matter of doubt; the connected problems are matters of detail, the solution of which is certain sooner or later. The same may be said of the members of any of the other great natural groups, such as the Arthropoda, the Annelida, the Echinodermata, etc.
It is different, however, when an attempt is made to connect two of the main divisions themselves. It is true enough that there is every reason to believe that the arthropod group has been evolved from the segmented annelid, and so the whole of the segmented invertebrates may be looked on as forming one big division, the Appendiculata, all the members of which will some day be arranged in orderly sequence, but the same feeling of certainty does not exist in other cases.
In the very case of the origin of the Appendiculata we are confronted with one of the large problems of evolution—the origin of segmented from non-segmented animals—the solution of which is not yet known.
The other large problem, perhaps the most important of all, is the question of the relationship of the great kingdom of the Vertebrata: from what invertebrate group did the vertebrate arise?
The great difficulty which presents itself in attempting a solution of this question is not so much, as used to be thought, the difficulty of deriving a group of animals possessing an internal bony and cartilaginous skeleton from a group possessing an external skeleton of a calcareous or chitinous nature, but rather the difficulty caused by the fundamental difference of arrangement of the important internal organs, especially the relative positions of the central nervous system and the digestive tube.