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Some things advance continuously, like a flood or a fire, which always make an end of A, eat and digest it, before they go on to B. Other things advance per saltum they do not silently cancer their way onwards, but lie as still as a snake after they have made some notable conquest, then when unobserved they make themselves up "for mischief," and take a flying bound onwards.
Such was the extraordinary leap taken by the Scottish clergy, into a power of which, hitherto, they had never enjoyed a fraction. It was a movement per saltum, beyond all that history has recorded.
Darwin therefore labors to show that it is not a special endowment, but an incidental acquirement. He does show that the sterility of crosses is of all degrees; upon which we have only to say, Natura non facit saltum, here any more than elsewhere. But, upon his theory he is bound to show how sterility might be acquired, through natural selection or through something else.
In each, the variety appears to have arisen in full force, and, as it were, per saltum; a wide and definite difference appearing, at once, between the Ancon ram and the ordinary sheep; between the six-fingered and six-toed Gratio Kelleia and ordinary men. In neither case is it possible to point out any obvious reason for the appearance of the variety.
The accumulated stores of ages, the riches of the Ninevite kings the "gold," the "silver," and the "pleasant furniture" of those mighty potentates, of which there was "none end" together with all the additions made to these stores by the Median monarchs, had fallen into his hands, and from comparative poverty he had come per saltum into the position of one of the wealthiest if not of the very wealthiest of princes.
Hence the canon of "Natura non facit saltum," which every fresh addition to our knowledge tends to make truer, is on this theory simply intelligible. We can plainly see why nature is prodigal in variety, though niggard in innovation. But why this should be a law of nature if each species has been independently created, no man can explain.
I have been surprised to find, making allowance for the instincts of animals having been but little observed except in Europe and North America, and for no instinct being known amongst extinct species, how very generally gradations, leading to the most complex instincts, can be discovered. The canon of "Natura non facit saltum" applies with almost equal force to instincts as to bodily organs.
The very men who most insisted that specific difference was the accumulation of differences so minute as to be often hardly, if at all, perceptible, could not see that the striking and baffling phenomena of design in connection with organism admitted of exactly the same solution as the riddle of organic development, and should be seen not as a result reached per saltum, but as an accumulation of small steps or leaps in a given direction.
Difficulties on the theory of descent with modification. Transitions. Absence or rarity of transitional varieties. Transitions in habits of life. Diversified habits in the same species. Species with habits widely different from those of their allies. Organs of extreme perfection. Means of transition. Cases of difficulty. Natura non facit saltum. Organs of small importance.
Colonel Humphreys, in fact, states that he was acquainted with only "one questionable case of a contrary nature." Here, then, is a remarkable and well-established instance, not only of a very distinct race being established 'per saltum', but of that race breeding "true" at once, and showing no mixed forms, even when crossed with another breed.
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