than in the crawling forms; in the naked rather than in the armored; in those of simple rather than in those of multiple organization—like the arthropods; in those with lateral rather than in those with oral limbs—like the mollusks; and finally in those with the smallest available number and most efficient character of limbs and other organs. This leads us to the vertebrates for the highest type that appeared in the waters of the earth, as the outcome of forms almost numberless in variety. In this we have an oval-shaped elongated animal, its organs of motion much the most effective of the many that had appeared in the progress of life, its vital organs unified and simplified to the greatest extent possible, its skeleton internal instead of external, used solely as a support, in no sense as an armor.
If we consider the fish in its most primitive varieties, we certainly seem led to the conclusion that it is the form to which the evolution of life would lead in any planet, as the basis of the higher development. In Amphioxus, for instance, we find the elongated bilateral animal simplified to an extraordinary degree; without external armor of any sort, with the simplest vital organs, with only the beginning of an internal skeleton, and with merely the suspicion of a fin, virtually a flattening of the skin. In this form we have the vertebrate reduced to its lowest terms, or the worm advanced to its highest. In the hag we find again a boneless and scaleless creature, with a sheath of cartilage to represent the backbone and with no organs of motion other than a fin-like flattening around the tail. Much the same may be said of the lamprey. From forms like these the fish seems to have developed, with all its subsequent variations.
The fish remains the highest form of water-developed life. It has made comparatively slight steps of progress during the immense interval since the paleozoic age. The limitations of its habitat seem to have checked the development of organic form at this stage. It can not be said that the evolution of life in the water has been in any sense restricted by deficiency of time or narrowness of variation. The variety of forms that have appeared is surprising when we consider the uniformity of conditions in the water, and are only to be accounted for as the result of a very active vital struggle. We find its simplest and least specialized higher result in Amphioxus, of which the ultimate result is the fish, beyond which, during millions of years, no progress has been made. And a full consideration of what has taken place on the earth strongly suggests that the oceanic evolution of life in any planet must have ended at some not dissimilar stage. Mentally it stands at a low level; and the whale, a mammal which has returned to the fishform, is as low as the fish in mental powers.
Life in water was the basis of life on the land. It could not originate there de novo, land conditions being unadapted to the early life
