700 ADDRESS TO THE DEPARTMENT OF ANATOMY 



which there is one. If a naturalist were to study the life-history, 

 or, in other words, the embryology of this form, this fact about 

 the pigment-spot would come to his notice, and he would be 

 justified, from the laws of heredity, in concluding that the species 

 was descended from an ancestor without a pigment-spot, because 

 a pigment-spot was absent in the young. Now, we may suppose 

 the transparent layer of skin above the pigment-spot to become 

 thickened, so as gradually to form a kind of lens, which would 

 throw an image of external objects on the pigment-spot. In this 

 way a rudimentary eye might be evolved out of the pigment- 

 spot. A naturalist studying the embryology of the form with 

 this eye would find that the pigment-spot was formed before the 

 lens, and he would be justified in concluding, by the same pro- 

 cess of reasoning as before, that the ancestors of the form he 

 was studying first acquired a pigment-spot and then a lens. We 

 may picture to ourselves a series of steps by which the simple 

 eye, the origin of which I have traced, might become more com- 

 plicated ; and it is easy to see how an embryologist studying the 

 actual development of this complicated eye would be able to 

 unravel the process of its evolution. 



The general nature of the methods of reasoning employed 

 by embryologists, who accept the Darwinian theory, is exempli- 

 fied by the instance just given. If this method is a legitimate 

 one, and there is no reason to doubt it, we ought to find that 

 animals, in the course of their development, pass through a series 

 of stages, in each of which they resemble one of their remote 

 ancestors; but it is to be remembered that, in accordance with 

 the law of variation, there is a continual tendency to change, and 

 that the longer this tendency acts the greater will be the total 

 effect. Owing, to this tendency, we should not expect to find a 

 perfect resemblance between an animal, at different stages of its 

 growth, and its ancestors; and the remoter the ancestors, the 

 less close ought the resemblance to be. In spite, however, of 

 this limitation, it may be laid down as one of the consequences 

 of the law of inheritance that every animal ought, in the course 

 of its individual development, to repeat with more or less fidelity 

 the history of its ancestral evolution. 



A direct verification of this proposition is scarcely possible. 

 There is ample ground for concluding that the forms from which 



