504 POPULAR SCIENCE MONTHLY. 



each breed with its own variety ; Ancon sheep have been observed 

 to keep together, separating themselves from the rest of the flock 

 when put into inclosures with other sheep ; " the female of the 

 dog " (according to Prof. Low, an authority on domesticated ani- 

 mals), " when not under restraint, makes selection of her mate, the 

 mastiff selecting the mastiff, the terrier the terrier, and so on " ; 

 pigeons pair, when choice is free, with their own kind ; flocks of 

 white and Chinese geese, even when associated by the breeder, 

 keep themselves distinct. The fact that organisms prefer to asso- 

 ciate and breed with their likes is also widely shown by the habits 

 of birds. Describing a molluscan fauna in the Sandwich Islands, 

 Mr. Gulick says: "We frequently find a genus represented in 

 several successive valleys by allied species, sometimes feeding on 

 the same, sometimes on different plants. In every such case the 

 valleys that are nearest to each other furnish the most nearly 

 allied forms ; and a full set of the varieties of each species presents 

 a minute gradation of forms between the more divergent types 

 found in the more widely separated localities." 



The recognition marks of insects, birds, and other animals 

 imply and involve the association of likes and the separation of 

 unlikes. The mimicry of one set of organisms by another, whether 

 the imitation be of color, shape, or both, is manifestly a means of 

 acquiring the superior ease of living which assimilation offers to 

 the imitating kind : the mimickers always occupy the same re- 

 gion as the species mimicked, and the resemblance wrought is 

 sometimes (Darwinism, page 25G) advantageous to both. All asso- 

 ciation of organisms involves likeness between them, and when, 

 by unequal conditions, unlikeness is set up in sufiicient degree, 

 that unlikeness involves dissociation of the unlike members of the 

 group, accompanied by the resistance which sterility offers to the 

 intercrossing of those members. The like individuals, moreover, 

 are only held together by the constant operation of assimilative 

 processes : one of these is the free intercrossing of all the associ- 

 ated members, whereby the individual is maintained true to the 

 average ; the other is the resultant tendency of the organism to 

 transmit and perpetuate only those characters due to influences 

 and conditions that are the common experiences of all the mem- 

 bers of a kind, as opposed to characters arising out of special 

 individual experiences. 



The group of animals, like the group of men, expels unlikes 

 that appear in its midst. Thus ants eject intruding members of 

 alien kinds ; geese and hens drive away strange birds of their own 

 species ; crows and rooks expel wrongdoers from their midst ; 

 domestic pigeons attack and badly wound sick, young, or fallen 

 birds ; beavers dissociate idle members from their colony ; a 

 wounded herbivorous animal returning to its companions is 



