54 THE SCOTTISH NATURALIST 



mind, viz., the principle of convergence and the chance 

 straggling of the parasites on to other hosts. For example, 

 is the Gallinaceous parasite on Penguins really Gallinaceous, 

 or has it had an independent origin, and merely converged 

 on the Gallinaceous genus Goniodes ? Mr Cummings con- 

 cluded by criticising the idea that " the parasites in their 

 evolution have necessarily lagged behind the evolution of 

 the host," and pointed out that " the parasites' evolution 

 sometimes appeared to progress /rt;/7'/rt:i-j-// with, and some- 

 times to progress beyond, that of the host." 



The next speaker, Mr T. Iredale, in the course of his 

 remarks emphasised the necessity for caution in regard to 

 the Petrels. Since, he said, these birds always breed in 

 colonies, with four or five distinct genera occurring in the 

 same breeding-district, there is a good chance of interchange 

 of parasites. " If ' stragglers' among the parasites are not 

 frequent under these circumstances, the students of Mallophaga 

 in the present connection have a very strong case in their 

 favour." Mr Iredale concludes that "the parasites will come 

 into use when there is a doubt about the systematic position 

 of a genus, and after considering the factors we get from the 

 birds' skins." 



In his reply Mr Harrison said that with regard to the 

 question of convergence he did not think any serious student 

 of these parasites who really examines the evidence closely 

 would insist very much upon convergence as a serious factor 

 in deciding relations in general. He added that we practically 

 knew nothing about Mallophaga yet. The question of 

 straggling, adduced by Mr Cummings, he admitted, but with 

 regard to the rate of evolution he maintained that the 

 general rate had been slower, while at the same time he 

 was willing to admit that there might be varying rates at 

 the ultimate ends of the evolutionary branches. 



