No. 545] 



ORIGIN OF UNIT CHAR A < / /•. /,' > 



253 



to the evolution of which the author has devoted twelve 

 years of investigation, assisted by Dr. W. K. Gregory. 

 As set forth in an earlier contribution 29 the genesis of 

 horns as rectigradations has been observed in four or 

 five distinct phyla of titanotheres. These phyla descend 

 independently from a single ancestor of remote geologic 

 age. Both in respect to new cusps on the teeth and new 

 horn rudiments on the skull there is observed what in 

 our ignorance may be called an ancestral predisposition 

 to the genesis of similar rectigradations. This predis- 

 position betrays the existence of law in the origin of cer- 

 tain new characters; it recalls a sagacious remark of 

 Darwin : 



. . . The principle formerly alluded to under the term of analogical 

 variation has probably in these eases often come into play; that is, the 

 members of the same class, althou.-li only distantly allied, have in- 

 herited so much in common in their constitution, that they are apt to 



obviously aid in the acquirement through natural selection of parts or 

 organs, strikingly like each other, independently of their direct in- 

 heritance from a common progenitor. 80 



Briefly, the origin of the titanothere horns is as fol- 

 lows: (a) from excessively rudimentary beginnings, i. e. f 

 rectigradations, which can hardly be detected on the sur- 

 face of the skull; (b) there is some predetermined law 

 or similarity of potential which governs their first exist- 

 ence, because (c) the rudiments arise independently on 

 the same part of the skull in different phyla at different 

 periods of geologic time; (d) the horn rudiments evolve 

 continuously, and they gradually change in form (i. e„ 

 allometrons) ; (e) they finally become the dominant char- 

 acters of the skull, showing marked variations of form 

 in the two sexes; (/) they first arise in late or adult 

 stages of growth, but are pushed forward gradually into 



-"The Four Inseparable Factors of Evolution. Theory of their Dis- 

 tinct and Combined Action in the Transformation of the Titanotheres, an 

 [Order Perissortaetyla, Sci<:nri . 



