98 IOWA ACADEMl OF SCIENCE Vol. XXVI, 1919 



It appears from the facts at hand that it is perfectly logical to 

 accept the condition as shown in fignres 7:1, 2; 8:3 as the normal, 

 both on account of its more frequent occurrence and again be- 

 cause it seems to be the condition around which abnormalities vary. 



As to the causes for these variations, it is beyond the realm of 

 this paper to determine. It is, however, of considerable ana- 

 tomical and developmental importance to throw some light upon 

 the subject on account of its frequency in mammalian animals. 

 McClure ('00) has discussed in detail abnormalities in the post 

 cava of the cat, as well as variations in the venous system of Didel- 

 phys. Hunt ('18) shows some interesting variations in the carotid 

 of the cat. Piersol in his Human Anatomy points out similar condi- 

 tions in the cceliac as well as in many other vessels in man. Besides 

 these, there are hosts of cases where other variations have been 

 noted in other mammals. 



McClure ('00) has suggested the possible causes for these ab- 

 normalities in the post cava of the cat as domestication, inbreeding, 

 disease, drugs and shocks. Concerning some of these there seems to 

 be some disagreement. Slomaker ('00) finds abnormalities in the 

 circulatory system of the common gray rabbit where domestication 

 as a cause is eliminated. McClure, later ('06), working with Didel- 

 phys, disproves at least one of his earlier conclusions as to the 

 cause. He says, "It is generally conceded that variations in the 

 venous system occur with greater frequency among domesticated 

 animals than among those living in the wild state ; an idea, however, 

 which is most certainly erroneous, as shown by the conditions met 

 with in Didelphys." It is my opinion that none of these five causes 

 suggested by McClure have any influence, or if so, they can hardly 

 be accepted as a determining element. 



The most general explanation, in many cases, as to the causes 

 of such variations is reversion. But Pearl ('08) has shown that 

 if we accept this explanation as to the cause we are simply begging 

 the question, while it raises a very important question concerning 

 natural selection. He says, "They all (abnormalities*) point to 

 the same conclusion ; namely, that onotogeny may take an entirely 

 new course, which in all probability has never appeared in the racial 

 history before, and yet immediately reach an adaptive end result." 

 "Natural selection by individual elimination clearly fails to explain 

 such adaptive morphogenesis." He states further, "It is suggested 

 that some form of internal selection offers a possible explanation for 

 such causes." 



•Writer's word in parenthesis. 



