282 VAHIA'IIOX I\ IMIJS OF PKNGUIX. 



kind, built uj) from individuals whose genelical relationships are 

 unknoVn, are unreliable as indicating evolutionary stages. It is, 

 however, manifest that, compared with the rest of the ribs, the 

 tenth pair is in a high state of hereditary variability within the 

 species. One is probably justified in assuming that the 

 variability is associated with the process of degeneration of the 

 j)articular pair ; but whether the process is actually in progress 

 at the present time can be determined only by taking the mean 

 of a sufficiently large number of measurements, and then 

 comparing it with a similar mean after an interval of time. 



That the variability is hereditary or germinal, in contrast to 

 fluctuating, may be iiiferred from the fact that the other ribs 

 show no individual variation of any significance. The first and 

 second vertebral ribs are incomplete with no sternal portions, 

 and the first is ^only about one-third the size of the second. Yet 

 in all the specimens examined they show no departures from 

 these conditions. One would expect that ancestrally these two 

 ribs were complete, and that their present reduced size is a 

 result of retrogressive evolution. Manifestly the ])rocess of 

 reduction has now ceased, and with regard to them there is 

 germinal uniformitj' throughout the species. The tenth pair 

 alone has a varied germinal representation, and the somatic 

 variability in the different individuals is evidence of this. 



Discussiox. 



Similar series of vaiiatioiis have been obtained in studying 

 certain characters of the ostrich, which may also with good 

 reason be assumed to be in a degenerative phase.* The feathers 

 of the wing, the second phalanx of the third finger, the claw on 

 the small outer fourth toe, and the scutellation of the big toe 

 all show different hereditary stages in retrogressive evolution, 

 when numbers of individuals are available for comparison. As 

 in the case of the tenth rib of the penguin, they are held to 

 represent different genetic stages in evolutionary degeneration 

 reached by the structures in question ; but whether the losses arc 

 still in progress at the present time is not determinable, and is 

 inmiaterial to the purpose of this paper. It is desired to emphasise 

 that wdiere a- structure is in a degenerative phase the individuals 

 making up the species are not necessarily at the same stage ; 

 some have retrogressed further than others; also, that though 

 interbreeding may be carried on, these stages do not lose their 

 identity by blending, nor result in a uniformity of the character 

 in question. 



Stages such as those presented by the penguin and ostrich 

 are the anatomical evidence of the way in which degenerative 

 evolution proceeds, and have to be accounted for in any complete 

 theory of evolution, a desideratum yet far from realisation. For 



" Methods of Degeneration in the Ostrich." Journal of Genetics, Vol. 

 IX, Jan., 1920. 



