sculptured. Many illustrations are found in the group of fishes. Coats of mail may be built up step by step, genus after genus, and then gradually modified or abandoned. In the Chætodon-Zanclus-Acanthurus-Balistes-Tetraodon-Diodon-Mola series of fishes, we have a high development and specialization of the spinous dorsal followed by its entire loss step by step, with that of the ventral fin also. The scales, at first normal, are specialized to lancets, bony plates, spinules, and then gradually reduced to mere prickles and finally lost.
In the Cirrhitus-Sebastes-Scorpcena-Cottus-Psychrolutes-Cyclopierus-Liparis-Paraliparis series, we have a higher and higher specialization of fins and scales, with the final loss of the latter and a reduction of the fins to their lowest terms. Similar series connect the typical sharks with the rays. Other series among fishes begin with specialized forms, but end in the degeneration seen in multiplied and unspecialized vertebræ and fin rays. The well-known horse series and the series of monkeys and apes—each genus in certain lines being progressively more anthropoid—may be considered in this connection. In fishes, many of these series may be clearly traced among forms still existing, the most primitive as well as the most recent or degenerate types being still represented in the sea. But, in a general way, when the geological series is known, it is found to run more or less parallel in time to the progressive changes we must imagine to have taken place. On this fact most recent paleontologists seem to be agreed.
While the phenomena exist and must be reckoned with, the causes are by no means clear. Perhaps the continuous operation of some form of selection may be conceivably potent in some cases. But the more primitive types still retain their vigor and abundance, and this fact must not be overlooked in our explanation. It may be noted that these series, as usually recognized, are made up of genera, each genus a step in some definite direction, with numerous diverging steps at the same time. But there is no evidence that the organisms in question individually vary in any one determinate direction, or that the tide of heredity is swayed by the forces which make for orthogenesis. Most naturalists disclaim any ideological implications in the terms used to describe these phenomena as 'determinate variation.' It is sufficient that it would seem that the line of succession of genera is determined by unknown causes, causes other than those potent in producing the divergence of heredity which we call the origin of species.
We may perhaps find some clue to these matters in the phenomena of analogous variation. Like conditions produce analogous results on forms of very different origin. Osborn notes that nature has often a very limited range of responses to external conditions. In upwards of a dozen different groups of fishes of widely different relationships, nature has developed gar-like jaws. In several different groups (Harriotta, Polyodon, Mitsukurina, Pegasus), she has produced forms
