Page:Popular Science Monthly Volume 48.djvu/546

504 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 animals), "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 associate 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 region as the species mimicked, and the resemblance wrought is sometimes (Darwinism, page 256) advantageous to both. All association of organisms involves likeness between them, and when, by unequal conditions, unlikeness is set up in sufficient 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 associated 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 members 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