y en la forma cortante de los dientes molares. En realidad, la dentadura difiere mucho: así, el perro tiene a cada lado de la mandíbula superior cuatro premolares y solo dos molares; mientras que el Thylacinus tiene tres premolares y cuatro molares. Los molares también difieren mucho en ambos animales en cuanto a tamaño relativo y estructura. La dentición adulta está precedida por una dentición de leche muy diferente. Cualquiera puede, por supuesto, negar que los dientes en ambos casos hayan sido adaptados para desgarrar la carne mediante la selección natural de variaciones sucesivas; pero si esto se admite en un caso, me resulta incomprensible que se niegue en el otro. Me alegra comprobar que una autoridad tan grande como el profesor Flower ha llegado a esta misma conclusión.
The extraordinary cases given in a former chapter, of widely different fishes possessing electric organs—of widely different insects possessing luminous organs—and of orchids and asclepiads having pollen-masses with viscid discs, come under this same head of analogical resemblances. But these cases are so wonderful that they were introduced as difficulties or objections to our theory. In all such cases some fundamental difference in the growth or development of the parts, and generally in their matured structure, can be detected. The end gained is the same, but the means, though appearing superficially to be the same, are essentially different. The principle formerly alluded to under the term of analogical variation has probably in these cases often come into play; that is, the members of the same class, although only distantly allied, have inherited so much in common in their constitution, that they are apt to vary under similar exciting causes in a similar manner; and this would obviously aid in the acquirement through natural selection of parts or organs, strikingly like each other, independently of their direct inheritance from a common progenitor.
Como las especies pertenecientes a distintas clases suelen haber sido adaptadas, mediante ligeras modificaciones sucesivas, para vivir bajo