466 



NATURE 



[Oct. 8, 1874 



states of health, and their occupations. Amongst the 

 latter are the habits and climate to which the offspring 

 is subject. Causes of this nature, many of which are very 

 incompletely understood, produce variations in the indi- 

 viduals of a species ; and as the offspring resembles its 

 parents, unless extra forces come into play to produce 

 differences, the peculiarities of each variety are capable of 

 transmission to the progeny. Thus, in course of time, 

 strongly marked varieties of a species are likely to 

 be developed; these give rise to others, until the de- 

 scendants are very different from their ancestral forms. 

 Time, however, besides continuing on the primitive stock 

 and developing new varieties, produces other effects with 

 equal certainty. Animals are dependent lor their exist- 

 ence on a certain supply of organised food. Those living 

 forms which furnish it have also been affected in a 

 manner similar to their destroyers ; like them, they have 

 varied, and they have tended to become more numerous 

 (the progeny in all cases being more numerous than the 

 parents). The area of occupation being necessarily limited, 

 and, as we are justified in assuming, fully stocked to com- 

 mence with, the multiplication of the progeny develops a 

 universal struggle for existence, one in which each indi- 

 vidual, for self-preservation sake, participates ; and in 

 which the weakest goes to the wall. As in other contests, 

 however, so in this, the race is not always to the swift, nor 

 the battle to the strong, for many of the destroying causes 

 are not those which are overt in their attacks. The sickly 

 blade of grass, under the shelter of an overshadowing stone, 

 protected from the browsing herd, fructities and repro- 

 duces itself, whilst its free-growing neighbours form a 

 delicious mouthful for the nibbling sheep. What amount 

 of strength or courage can protect the leader of a flock 

 from the ravages of an intestinal parasite ? or prevent the 

 largest individual of a flight of birds from being the most 

 likely, on account of its greater superficial area, to be killed 

 by a random gun-shot .'' Specialisation of function to resist 

 special attack or to acquire special advantage, is, therefore, 

 on account of the struggle for existence in conjunction 

 with the tendency to vary, a factor of vitality. Specialisa- 

 tion in many directions is elaboration and progress so 

 called ; and as man possesses this in the most marked 

 degree, he is considered to be the furthest removed from 

 the living monad which gave him origin. 



The pedigree of vitality is evidently, therefore, the 

 greatest problem of biology ; for a full comprehension of 

 it includes all the minor details of the science. How is 

 this to be arrived at .'' From any collection of people 

 which comprises nearly all the living representatives of a 

 family, it is not difficult to obtain a large amount of infor- 

 mation with regard to the ancestry of that family by oral 

 interrogation. This will bo facilitated by classing to- 

 gether in groups those of equal kinship, placing in the 

 same sections brothers and sisters, in larger divisions 

 those who are first cousins, and so on. It will not be 

 hard to find who were the grandparents of each, some 

 probably being present ; the great-grandparents of most 

 will have only been personally known to the older ; and 

 more distant relations of the same line, by hearsay alone. 

 Pursuing the investigation, the linking of each retrograde 

 step will be found more difficult, and the difficulty of iden- 

 tifying the ancestor common to them all will be almost 

 insurpassable. ^Vhen an old family has very few living 

 representatives or none at all, the facilities for studying 

 it will be proportionately diminished. 



In zoology the method of investigation for the purpose 

 of classification is very similar. Instead of direct inter- 

 rogation, answers are arrived at by an appeal to facts of 

 existing structure. Similarity in habits, distribution, and 

 external characters separate off closely related forms from 

 their more distant allies. To solve the more difficult 

 problems of less intimate relationships, recourse must be 

 had to internal characters in addition ; to points of differ- 

 ence in osteological and soft-part anatomy, many of which 



can only be arrived at by prolonged dissection and the 

 employment of every available opportunity. 



The difficulty of appreciating the relative value of 

 differences in any group of animals that is forming the 

 subject of investigation, that of separating the realisation 

 of the characters themselves, independently from the 

 words necessary to express them, has led me in the course 

 of my dissections to adopt a method of formulating my 

 results in a manner which at once places them in a form 

 available for ready comparison, and in an order of relative 

 significance ; in fact as rational formula:', which differ in 

 arrangement according to the phases of my general ideas. 

 An example of the application and the applicability of this 

 method may not be without interest, and this I will draw 

 from the sub-order Psittaci, the Parrots. 



The parrots form a well-marked, easily distinguishable 

 group, with no outlying doubtful genera ; and as with 

 many other well-marked groups, such as the Rodents 

 amongst mammals, and the Umbellifera; amongst phane- 

 rogamic plants, the minor divisions are not so easily 

 determinable. In fact, there is a very great uniformity 

 in all the external and internal characters throughout the 

 sub-order. There are, however, a few points in which 

 they present variations, those best known being (i) in 

 the vessels of the neck, (2) in the ambiens muscle, (3) in 

 the furcula, and (4) in the oil-gland. I will notice each 

 of these points shortly. 



Firstly, with regard to the vessels of the neck. In 

 most of the higher animals an artery, the carotid, runs 

 up each side of the neck to supply blood from the heart 

 to the head. In birds these vessels generally run in the 

 middle line of the front of the neck, side by side and in 

 contact. In some parrots, and in them only, whilst the 

 right carotid pursues its usual course, the left, leaving 

 its fellows, runs separately at the side along with the left 

 pneumogastric nerve. In several groups of birds the 

 right carotid is absent, the left alone remaining in its 

 normal position. This is the case with one genus of 

 parrots. Secondly, the little long and slender muscle, 

 the ambiens, whose tendon in its unique course obliquely 

 traverses the front of the knee capsule, is absent in some 

 parrots, being present in others. Thirdly, the furcula or 

 merrythought, which unites the two shoulders by an 

 osseous bow, may be present or absent. Fourthly, the 

 oil-gland, situated just over the tail, is wanting in some 

 genera. 



Omitting for the time being the case, which amongst 

 the parrots is found only in the genus Cacatua proper, in 

 which the left carotid alone is present, there are sixteen 

 possible combinations of the four characters under con- 

 sideration, of which six are found to exist. They are the 

 following r — 



1. The carotids are normal ; the ambiens is absent ; 



the furcula is present, as is also the oil-gland. — 



(PAL.liORNITHIN/E.) 



2. The carotids are normal ; the ambiens is absent, 



as is the furcula, and the oil-gland is present. — 



(STRIN'GOPIN.E.) 



3. The carotids run abnormally ; the ambiens is present, 



as is the furcula and the oil-gland. — (AriN/E.) 



4. The carotids run abnormally ; the ambiens is ab- 



sent ; the furcula and the oil-gland are present. — 



(PVHRRURIN.F. ) 



5. The carotids run abnormally ; the ambiens is ab- 



sent, as is the furcula ; the oil-gland is present.— 

 (Pl.\tvcercin.e.) 



6. The carotids run abnormally ; the ambiens is ab- 



sent ; the furcula is present ; the oil-gland is 



absent. — (Chrvsotin.e.) 

 The facility for comparison afforded by a formulation 

 of these results will be evident from an inspection of the 

 following Table, in which the presence or absence of 

 structures is represented by the signs + or — ; in which 

 the normal condition of the carotid arteries is indicated. j?. 



ed^ 



