Sept. 2, 1880] 



NATURE 



417 



if we imagine the masses of iron, nickel, and magnesium 111 

 the sun to retain even a slight degree of magnetic power m 

 their gaseous state— and we know from the researches of Fara- 

 day that gases are some of them magnetic— we have a suflicient 

 cause for all our terrestrial magnetic changes, for we know that 

 these masses of metal are ever boiling up from the lower and 

 hotter levels of the sun's atmosphere to the cooler upper 

 reo-ions, where they mu'^t again form cloudi to throw out their 

 light and heat, and to ab,orb the light and heat coming from the 

 hotter lower regions ; then they become condensed and are 

 drawn again back tow ards the body of the sua, so forming those 

 remarkable dark spaces or sun-spots by their downrush towards 

 the lower levels. 



In these vast changes, which we know from the science of 

 energy must be taking place, bat of the vastness of which we 

 can have no conception, we have abundant cause for these 

 magnetic changes which we observe at the same instant at distant 

 points on tlie surface of the earth, and the same cause acting by 

 induction on the magnetic matter within and on the earth may 

 well produce changes in the magnitude or in the direction of its 

 total magnetic force. These magnetic changes on the earth will 

 influence" the declination needles at different places, and will 

 cause them to be deHected ; the direction of the deflection must 

 depend on the situation of the earth's magnetic axis or the 

 direction of its motion \\ ich regard to the stations where the 

 observations are made. Thus both directly and indirectly we 

 may find in the Sun not only the cause of diurnal magnetic varia- 

 tions, but also the cause of these remarkable magnetic changes 

 and disturbances over the surface of the Earth. 



SECTION D 



DepaHinent of Anatomy and Physiology 



Address by F. M. E.\lfour, M.A., F.R.S., Vice-President 

 OF THE Section 



In the spring of the present year Prof. Huxley delivered an 

 address at the Royal Institution, to which he gave the felicitous 

 title of " The Coming of Age of the Origin of Species." It is, 

 as he pointed out, twenty-one years since Mr. Darwin's great 

 work was published, and the present occasion is an appropriate 

 one to review the effect which it has had on the progress of 

 biological knowledge. 



There is, I may venture to say, no department of biology the 

 growth of which has not been profoundly influenced by the Dar- 

 winian theory. When Messrs. Darwin and Wallace first 

 enunciated their views to the scientific world, the facts they 

 brought forward seemed to many naturalists insufficient to sub- 

 stantiate their far-reaching conclusions. Since that time an 

 overw'helming mass of evidence has, however, been rapidly 

 accumulating in their favour. Facts which at first appeared to 

 be opposed to their theories have one by one been shown to 

 afford strildng proofs of their truth. There are at the present 

 time but few naturalists who do not accept in the main the Dar- 

 winian theory, and even some of those who reject many of 

 Darwin's explanations still accept the fundamental position that 

 all animals are descended from a common stock. 



To attempt in the brief time which I have at my disposal to 

 trace the influence of the Darwinian theory on all the branches 

 of anatomy and physiology would be 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 revolutionised, 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. The law of heredity is familiarly exemplified by the 

 well-knoun fact that offspring resemble tlicir parents. Not 

 only, however, do the offspring belong to the same species as 

 their parents, but they inheiit the individual peculiarities of their 

 parents. It is on this that the breeders of cattle depend, and it 

 is a fact of every-day experience amongst ourselves. A further 

 point with reference to heredity to which I must call your atten- 

 tion 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 off- 



spring 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 hereility. It asserts that the resemblance which offspring 

 bear to their parents is never exact. The contradiction between 

 the two laws is only apparent. All variations and modifications i n 

 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 ou its parents ; and the \a.v; of heredity really asserts that 

 the offspring and parent would resemble each other if their en- 

 vironments were the same. Since, however, this is never the 

 case, the offspring always differ 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 applica- 

 tion will best be made apparent by taking a concrete example. 

 Let us suppose a spot on the surface of some very simple or- 

 ganism to become, at a certain period of life, pigmented, and 

 therefore to be especially sensitive to hght. In the offspring of 

 this form the pigment-spjt will reappear at a corresponding 

 period ; and there will therefore be a period in the life of the 

 offspring 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, 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 pig- 

 ment-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 w ould 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 em- 

 bryology 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 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 

 i-teps 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 studying the actual development of this complicated 

 eye would be able to unravel the process of its evolution. 



Tlie general nature of the methods of reasoning employed by 

 embryotogists, 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 

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

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

 cestors ; 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 Owtng to this tendency we should not expect to find a 

 perfect resemblance between an animal, at diff'erent stages of Us 

 Growth and its ancestors ; and the remoter the ancestors, the 

 Tess close ou-^ht 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 fidehty 

 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 

 e.xi-tin- animals are descended have in most instances perished : 

 and aUhouo-h 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 

 micht have been hoped. . 



While for the reasons just stated, it is not generally possible 

 to prove'by direct observation that existing forms in their em- 

 brvonic state repeat the characters of their ancestors, there is 

 another method by which the truth of this proposition can be 

 approximately verified. , , .u 



A comparison of recent and fossil forms shows that there are 

 actually livinc^ at the present day representatives of a consider- 

 'able proportion of the groups which have in previous times 

 existed on the gl ibe, and there are therefore forms allied to the 

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



