ZOOLOGY. 



the admission of the special creation of a single 

 progenitor for each species, or of two, according 

 as the sexes may be united or distinct. As 

 Edward Forbes expresses it (Natural History of 

 the British Seas, page 8) : ' Every true species 

 presents in its individuals certain features, specific 

 characters, which distinguish it from every other 

 species ; as if the Creator had set an exclusive 

 mark or seal upon each type.' From time to 

 time, however, naturalists of high eminence, and 

 among them the celebrated French zoologist 

 Lamarck, dissented from this view, and contended 

 that the weight of evidence was rather in favour 

 of a gradual development of the whole animal 

 series from the simpler to the more complex ; a 

 development comparable in certain respects to the 

 development of the individual from the germ to 

 maturity. 



The ' doctrine of evolution ' has assumed various 

 forms more or less plausible. It will be sufficient 

 here to give a slight sketch of the latest of those 

 speculations, the one which has received the 

 widest acceptance, and exerted the most powerful 

 influence upon the current of human thought the 

 ' Darwinian theory ' of the origin of species by 

 natural selection. 



This speculation for it cannot be said to have 

 assumed anymore definite form is based upon 

 the known phenomena of variation. 



It is very evident that offspring have a ten- 

 dency to resemble their parents very closely in 

 form and structure ; and this resemblance, in all 

 cases within our experience, brings parent and 

 offspring within the limit of the same species. 



It is equally evident that offspring have a 

 tendency to differ individually from their parents 

 to a certain degree, but this variation is slight, 

 involving no specific distinction ; and in no case, 

 so far as we are at present aware, interfering with 

 the power of reproduction, or with the fertility of 

 the succeeding generation. 



The range of variation of the individuals com- 

 posing a species is thus very definite, and is 

 limited by the circumstances under which the 

 group of individuals is placed. Except in man 

 and in domesticated animals, in which it is 

 artificially increased, this individual variation is 

 usually so slight as to be inappreciable, except to 

 a practised eye ; and any extreme variation which 

 passes the natural limit in any direction clashes 

 in some way with surrounding circumstances, and 

 is dangerous to the life of the individual. The 

 normal line or ' line of safety ' for any species lies 

 midway between the extremes of variation. 



If at any period in the history of a species the 

 conditions of life of a group of individuals of the 

 species be gradually altered ; if, for example, the 

 temperature of the region which they inhabit be 

 changed, or if there be an alteration in the amount 

 or kind of food, with the gradual change of cir- 

 cumstances, the limit of variation is contracted in 

 one direction and relaxed in another ; it becomes 

 more dangerous to diverge towards one side, and 

 more desirable to diverge towards the other, and 

 the position of the lines limiting variation is thus 

 altered. The 'normal line' along which the 

 specific characters are most strongly marked is 

 consequently slightly deflected, some characters 

 being more strongly expressed at the expense of 

 others. This deflection, carried on for ages in 

 the same direction, must eventually carry the 



divergence of the varying race far beyond any 

 limit within which we are in the habit of admit- 

 ting identity of species. Those individuals in 

 which the favourable variation is most marked 

 will have the advantage in this struggle for life ; 

 will be the stronger and the more comely, and 

 will naturally select one another as partners for 

 the perpetuation of the race; thus continuing 

 and exaggerating the favourable peculiarity from 

 generation to generation. 



We must admit that variation is a vera causa, 

 capable, within a limited period, under favourable 

 circumstances, of converting one species into 

 what, according to our present ideas, we should 

 be forced to recognise as a different species. 

 And such being the case, it is, perhaps, conceiv- 

 able that during the lapse of a period of time- 

 still infinitely shorter than eternity variation 

 may have produced the entire result. There is 

 no doubt great difficulty in imagining that, com- 

 mencing from the simplest living being, the pres- 

 ent state of things in the organic world has 

 been produced solely by the combined action of 

 ' atavism ' and ' variation ; ' and many are still 

 inclined to believe that some other law than the 

 ' survival of the fittest ' must regulate the existing 

 marvellous system of extreme and yet harmonious 

 modifications. Still, we are probably justified in 

 saying that there is now scarcely a single com- 

 petent general naturalist who is not prepared to 

 accept some form of the doctrine of evolution. 

 But the process must be infinitely slow. It is 

 difficult to form any idea of ten, fifty, or a 

 hundred millions of years, or of the relation which 

 such periods bear to changes taking place in the 

 inorganic world. But if it be possible to imagine 

 that this marvellous manifestation of Eternal Power 

 and Wisdom involved in living nature can have 

 been worked out through the law of ' descent with 

 modification ' alone, we shall certainly require the 

 longest row of figures which the inexorable phy- 

 sicists can afford. 



The origin of species by descent with modifica- 

 tion is as yet only an hypothesis. During the 

 whole period of recorded human observation, not 

 one single instance of the change of one species 

 into another has been detected ; and, singular to 

 say, in successive geological formations, although 

 new species are constantly appearing, and there 

 is abundant evidence of progressive change, no 

 single case has yet been observed of one species 

 passing through a series of inappreciable modi- 

 fications into another. 



To a number of species agreeing in most points 

 of importance, and having kindred characters, 

 and these characters appealing broadly to the 

 senses, the term genus meaning a kind is 

 applied. From the arrangement of species of 

 animals in genera has sprung the modern system 

 of zoological nomenclature, first adopted by the 

 celebrated Linnaeus. This system has received 

 the name of the binomial system, from the fact 

 that each animal receives two names, one belong- 

 ing to itself alone, and the other which it possesses 

 in common with the other species of the genus 

 under which it is placed. Thus, the genus Equus 

 contains the Ass, Horse, and Zebra as species. 

 To all, the generic name Equus is applied ; but a 

 second or specific name is added to each, to dis- 

 tinguish it from all other species of the genus : 

 thus, the Horse is called Equus caballus; the 



