160 Darwin and his Teachings. [ April, 
photographs beyond those of his pen, nor of any witnesses besides 
himself. 
And there are certain phenomena connected with the nature of 
the lower animals and of man which appear at that time to have 
made a remarkable impression upon the young traveller. One of 
these was the “tendency to variation,” by slow degrees, m some 
species. Speaking, for example, of a certain venomous snake in 
Patagonia, a Trigonocephalus, he says— 
“ Cuvier, in opposition to some other naturalists, makes this a sub- 
genus of the rattlesnake, and intermediate between it and the viper. 
In confirmation of this opinion I observed a fact, which appears to 
me very curious and instructive, as showing how every character, 
even though it may be in some degree independent of structure, 
has a tendency to vary by slow degrees. The extremity of the tail 
of this snake is terminated by a point which is slightly enlarged, 
and as the animal glides along it constantly vibrates the last inch, 
and this part striking against the dry grass and brushwood pro- 
duces a rattling noise, which can be distinctly heard at the distance 
of six feet. As often as the animal was irritated or surprised, its 
tail was shaken, and the vibrations were extremely rapid. Even as 
long as the body retained its irritability a tendency to this habitual 
movement was evident. This Trigonocephalus has, therefore, in some 
respects, the structure of a viper, with the habits of a rattlesnake ; the 
noise, however, being produced by a simpler device.”* 
Another phenomenon, illustrating in a conspicuous manner his 
subsequent law concerning the preservation of favoured races and 
the extinction of others,—a most important feature in his theory 
of natural selection,—presented itself to his notice durimg his stay 
in Banda Oriental. It was connected with a peculiar breed of 
cattle in that country, and we will extract his account of it :— 
“Don F. Muniz, of Luxan, has kindly collected for me all the in- 
formation he could respecting this breed. From his account it seems 
that about eighty or ninety years ago they were rare, and kept as 
curiosities at Buenos Ayres. The breed is universally believed to 
have originated amongst the Indians southward of the Plata; and it 
was with them the commonest kind. Even to this day, those reared 
in the provinces near the Plata show their less civilized origin, in 
being fiercer than common cattle, and in the cow easily deserting her 
first calf if visited too often, or molested. It is a singular fact, that 
an almost similar structure to the abnormal one of the niata breed, 
characterizes, as Iam informed by Dr. Falconer, that great extinet 
ruminant of India, the Sivatherium. The breed is very true, and a 
niata bull and cow invariably produce niata calves. A niata bull with 
a common cow, or the reverse cross, produces offsprings having an in- 
termediate character, but with the niata characters strongly displayed. 
According to Sefior Muniz, there is the clearest evidence, contrary to 
* «Journal of Researches,’ pp. 96-7. 
