GENETIC CHANGE AND EVOLUTIONARY CHANGE 411 



tion might not be directly involved in the matter of horn length. Theoreti- 

 cally, the time might come when the horn would become so large as to be a 

 positive handicap. Then natural selection would tend to eliminate posses- 

 sors of the oversized horns. Among prehistoric animals some instances are 

 observed of animals possessing horns and other structures so huge that 

 they seem to have passed the point of maximum usefulness. The huge 

 spread of antlers possessed by the Irish stag affords a case in point. 

 Whether or not the excessive antler development contributed to the 

 animal's extinction is still the subject of lively debate. But we may feel 

 reasonably confident that the tendency of these big stags to develop dispro- 

 portionately huge antlers represented the culmination of such a process of 

 differential growth as we have been discussing. (See Huxley, 1932, for 

 discussion of allometry of antlers. ) 



We may well mention in passing that the observed tendency of a part 

 or organ to change progressively in size is sometimes given as an example 

 of what is called orthogenesis, evolution in a straight line. In such a termi- 

 nology the steady increase in length of horn in our rhinoceroslike animal 

 would form an example of an orthogenetic series. Orthogenesis as a de- 

 scriptive term indicating the occurrence of progressive changes is some- 

 times useful. But, unfortunately, it has at times been endowed with an oc- 

 cult meaning and presented as an evolutionary force in its own right, as 

 though there were some inner force in animals tending to cause them to 

 evolve in straight lines. Further discussion of orthogenesis is not appro- 

 priate here; readers are referred to Jepsen (1949) and Simpson (1953a), 

 for more extended treatment of the controversial subject. We recall 

 that the "line" leading from Hyracotherium to Equiis (Fig. 10.7, p. 204) 

 frequently cited as an orthogenetic line, was in reality singled out for atten- 

 tion from among many other lines actually existing. We may quote with 

 approval Simpson's conclusion that much apparent orthogenesis is "a prod- 

 uct rather of the tendency of the minds of scientists to move in straight 

 lines than of a tendency for nature to do so" and note further that at least 

 some of the progressive series which are observed are explicable as the 

 result of differential growth rates. Other progressive series are explicable 

 as the result of operation of natural selection on organisms living in a stable 

 environment or an environment that is changins with a constant trend 

 (e.g., becoming increasingly dry). Under such conditions natural selection 

 promotes more and more perfect adaptation to that environment and the 

 resulting changes may take the form of a progressive series. Natural selec- 

 tion operating in this manner is sometimes called "orthoselection." (See 

 also "Directive Forces in Evolution," pp. 496-500.) 



