VARIATIONS IN ANIMALS AND PLANTS 501 



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 Chcstodon-Zanclus-Acan- 

 thurus-Balistes-Tetraodon-Diodon-JIola 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-Cyclopier- 

 us-Liparis-Paraliparis series, we have a higher and higher specializa- 

 tion 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 

 vertebra? 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 repre- 

 sented 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 progress- 

 ive changes Ave 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 ques- 

 tion 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 pro- 

 ducing 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 up- 

 wards of a dozen different groups of fishes of widely different relation- 

 ships, nature has developed gar-like jaws. «In several different groups 

 (Harriotta, Polyodon, Mitsukurina, Pegasus), she has produced forms 



