638 REPORT — 1880. 



"between an animal, at different stages of its growth, and its ancestors ; and the re- 

 moter 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 existing animals are descended 

 have in most instances perished ; and although there is no reason why they should 

 not have been preserved in a fossil state, yet, owing to the imperfection of the 

 geological record, palaeontology is not so often of service as might have been hoped. 



While for the reasons just stated, it is not generally possible to prove by direct 

 observation that existing forms in their embryonic state repeat the characters of 

 their ancestors, there is another method by which the truth of this proposition can 

 be approximately verified. 



A comparison of recent and fossil forms shows that there are actually li-\-ing at 

 the present day representatives of a considerable proportion of the gxoups which have 

 in previous times existed on the globe, and' there are therefore forms allied to the 

 ancestors of those living at the present day, though not actually the same species. 

 If therefore it can be shown that the embryos of existing forms pass through 

 etao-es in which they have the characters of more primitive groups, a sufficient 

 proof of our proposition will have been given. 



That such is often the case is a well-known fact, and was even known before 

 the publication of Darwin's works. Von Baer, the gi-eatest embryologist of the 

 century, who died at an advanced age but a few j'ears ago, discussed the proposition 

 at considerable length in a work published between the years 1830 and 1840. He 

 came to the conclusion that the embryos of higher forms never actually resemble 

 lower forms but only the embryos of lower forms ; and he further maintained that 

 such resemblances did not hold at all, or only to a very small extent, beyond the 

 limits of the laro'er groups. Thus he believed that, though the embryos of Verte- 

 brates mio'ht ao-ree amongst themselves, there was no resemblance between them and 

 the embryos of any invertebrate group. We now know that these limitations of 

 Von Baer do not hold good, but it is to be remembered that the meaning notu 

 attached by embryologists to such resemblances was quite unknown to him. 



These preliminary remarks will, I trust, be sufficient to demonstrate how com- 

 pletely modern embryological reasoning is dependent on the two laws of inheritance 

 and variation, which constitute the keystones of the Darwinian theory. 



Before the appearance of the ' Origin of Species ' many very valuable embryo- 

 loo-ical investio-ations were made, but the facts discovered were to their authors 

 merely so many ultimate facts, which admitted of being classified, but could not 

 be explained. No explanation could be offered of why it is that animals, instead of 

 developing in a simple and strfughtforward way, undergo in the course of their 

 growth a series of complicated changes, during which they often acquire organs 

 which have no function, and which, after remaining visible for a short time, 

 disappear without leaving a trace. 



No explanation, for instance, could be offered of why it is that a frog in the 

 course of its o-rowth has a stage in which it breathes like a fish, and then why it is 

 like a newt with a long tail, which gi-adually becomes absorbed, and finally disappears. 

 To the Darwinian the explanation of such facts is obvious. The stage when the 

 tadpole breathes by gills is a repetition of the stage when the ancestors of the frog had 

 not advanced in the scale of development beyond a fish, while the newt-like stage 

 implies that the ancestors of the frog were at one time organised very much like 

 the newts of to-day. The explanation of such facts has opened out to the embryo- 

 logist quite a new series of problems. These problems may be divided into two 

 main gi-oups, technically known as those of phjlogeny and those of organogeny. 

 The problems of phylogeny deal with the genealogy of the animal Idngdom. A 

 complete genealogy would form what is known as a natural classification. To 

 attempt to form such a classification has long been the aim of a large number of 

 naturalists, and it has frequently been attempted without the aid of embryology. 

 The statements made in the earlier part of my address clearly show how great an 



