240 DARWINISM TO-DAY. 



basis is defined by the presence of some sort of a physical 

 barrier. It is extremely rare to find two subspecies inhabit- 

 ing or breeding in exactly the same region. When such 

 appears to be the case, there is really some difference in 

 habit or in habitat ; the one form lives on the hills, the other 

 in the valleys ; the one feeds on one plant, the other on 

 another ; the one lives in deep water, the other along the 

 shore. There can be no possible doubt that subspecies are 

 nascent species, and that the accident of intergradation in 

 the one case and not in the other implies no real difference 



in origins. 



'To the general rule that closely allied species do not live 

 together there exist partial exceptions. It may be well to 

 glance at some of these, for no rule is established until its 

 exceptions are brought into harmony with the phenomena 

 which illustrate the rule." (Here Dr. Jordan details the 

 facts of distribution in three cases from among the fishes, 

 which apparently form exceptions to his general rule). 



As an example of the effects of an unusual and interesting 

 phase of isolation I may refer to the conditions noted con- 

 cerning'- the distribution and species distinction 

 Effects of 



isolation in the of the Mallophaga, a group of small wingless 

 Mallophaga. j nse ct parasites on birds and mammals. These 

 parasites live for their whole lives among the feathers or 

 hair of their hosts, and while able to run swiftly are unable 

 to fly and thus to migrate freely from bird to bird. 



'There are to be noted various results of the influence on 

 the taxonomy of the Mallophaga of the peculiar conditions 

 of their parasitic life. While the uniformity and persistence 

 of the conditions under which the life of the parasites is 

 passed tend to preserve with little change the species types, 

 the peculiar isolation, often pretty complete, of groups of 

 individuals of a parasite species on individual birds of the 

 host species and the consequent close breeding, tend to 

 foster and fix those inevitable slight variations always mani- 



