T1{ANSACTI0^S OF SECTIO.V D. — DEPT. AXATOMY AND niYSIOLOGY. 637 



wholly impossible, and I shall confine myself to an attempt to do so for a small 

 section only. There is perhaps no department of Biology which has been so revolu- 

 tionised, if I may use the term, by the theory of animal evolution, as that of 

 Development or Embryology. The reason of this is not far to seek. According to 

 the Darwinian theory, the present order of the organic world has been caused by 

 the action of two laws, known as the laws of heredity and of variation. Tha 

 law of heredity is familiarly exemplified by the well-known fact that offspring 

 resemble their parents. Not only, however, do the ofi'spring belong to the same 

 species as their parents, but they inherit the individual peculiarities of their parents. 

 It is on this that the breeders of cattle depend, and it is a fact of every-day ex- 

 perience amongst ourselves. A further point with reference to heredity to which I 

 must call your attention is the fact that the characters, which display themselves 

 at some special period in the life of the parent, are acquired by the offspring at a 

 corresponding period. Thus, in many birds the males have a special plumage 

 in the adult state. The male oftspring is not, however, born with the adult plumage, 

 but only acquires it when it becomes adult. 



The law of variation is in a certain sense opposed to the law of heredity. It 

 asserts that the resemblance which oftspring bear to their parents is never exact. 

 The contradiction between the two laws is only apparent. All variations and 

 modifications in an organism are directly or indirectly due to its environments ; 

 that is to say, they are either produced by some direct influence acting upon the 

 organism itself, or by some more subtle and mysterious action on its parents; and 

 the law of heredity really asserts that the oftspring and parent would resemble each 

 other if their environments were the same. Since, however, this is never the 

 case, the oftspring always difter to some extent from the parents. Now, according 

 to the law of heredity, every acquired variation tends to be inherited, so that, by a 

 summation of small changes, the animals may come to differ from their parent 

 stock to an indefinite extent. 



We are now in a position to follow out the consequences of these two laws in 

 their bearing on development. Their application will best be made apparent by 

 taking a concrete example. Let us suppose a spot on the surface of some very 

 simple organism to become, at a certain period of life, pigmented, and therefore 

 to Be especially sensitive to light. In the oftspring of this form, the pigment-spot 

 will reappear at a corresponding period ; and there will therefore be a period in the 

 life of the oftspring during which there is no pigment-spot, and a second period in 

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

 words, the embryology of this form, the 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 tliickeued, so as gradually to fovm 

 a kind of lens, capable of throwing an image of external objects on the pigment- 

 spot. In this way a rudimentary ej'e might be evolved out of the pigment-spot. 

 A naturalist studying the embryology of the form with this 650 would find that 

 the pigment-spot was formed before the lens, and he would be justified in con- 

 cluding, by the same process 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 complicated ; and it is easy to see how an embryologist 

 studpng 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 exemplified 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, iu 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 remem- 

 bered 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 efl'ect. 

 Owing to this tendency, we should not expect to find a perfect resemblance 



