614 



SPECIES 



SPECIFIC DENSITY 



there are many cases of fertile hybrids between 

 different opecies. We can no longer talk as if the 

 mule were the only known hybrid. See HYBRID. 



Or if we turn to the systematic treatises which 

 classify plants anil animals, anil compare half a 

 dozen 'of them, we find ample evidence of the 

 elasticity of the conception of species. ','""' hom- 

 ines tot sententiit. Thus, as Haeckel notices, one 

 botanist enumerates 300 German species of the 

 common Composite, Hieracium ; another reduces 

 them to 106, another to 52, another to about a 

 score ! Bechstein said that there were 367 species 

 of birds in Germany, but according to Reichenbach 

 there are 379, acco'rding to Meyer and Wolf 406, 

 according to Pastor BreTim 900 ! But Haeckel him 

 self supplies the best example, for in hi- important 

 monograph on Calcareous Sponges he admits thai 

 as the naturalist likes to look at the problem 

 there are 3 species, or 21, or 289, or 591 ! We are 

 told that species are groups of individuals agree- 

 ing in essential characters which remain constant 

 from generation to generation. But what are essen- 

 tial characters ? and how much constancy is demon- 

 strable ? 



We cannot forget, for instance, how one species 

 may include forms so very different as Shetland 

 pony, hunter, and dray horse ; or as poodle, bull- 

 dog, and greyhound ; or as carrier, pouter, and 

 fantail ; or as cabbage, cauliflower, and Brussels 

 sprouts. Yet it is probable that in each of these 

 four groups the diverse forms have been derived 

 from the same ancestral wild species, and in each 

 case the diverse forms are connected by inter- 

 mediate stages. 



In short, the fact is that there is no such thing 

 as species. Individuals are real ; but a species is a 

 subjective conception. It is based on structural 

 resemblances between individuals, and the degree 

 of importance attached to these depends, as we 

 have seen, on the mind of the observer, or is, in 

 other words, entirely relative. 



But while there can be no perfectly strict defini- 

 tion of a species in terms of morphology, it may be 

 asked, where is there a possible one in terms of 

 physiology, in terms of functional peculiarities 

 about which there can be no dispute? At one 

 time it almost seemed that there might be some 

 solution in terms of fertility and infertility. But, 

 although thi* distinction is certainly helpful and 

 very important, it also breaks down. It is not of 

 course doubtful that species have physiological 

 peculiarities ; lions differ from tigers in habit and 

 chemical composition a- well as in form and struc- 

 ture ; but every individual has also its peculiarities 

 chemical as well as personal ; the difficulty is to 

 decide when these peculiarities are ini|K>rtant 

 enough to make it useful to give a precise name to 

 their possessors. 



As to the practical question of determining 

 species it should lie borne in mind that the differ- 

 ences between one form and another are often very 

 market), and that the gap lietween related forms is 

 not always bridged by any unbroken series. The 

 New Zealand lizards (Hatteriti jiiinrtiifii) not only 

 form 11 species, hut are the sole living representa- 

 tive- of mi order, or of a class; and the same 

 may IM- said ol" the lancelet and other forms. It is 

 \viili those organisms of which there are very many 

 more or less different forms that there is real diffi- 

 culty with Bacteria, Algu-, Protoxoa, Sponges, 

 Crustaceans, insects. Fishes, Minis, anil so on. In 

 Midi cases the naturalist who admits that species 

 is but a relative conception, and who, as an evolu- 

 tionist, recognises the variability of species and the 

 links of relationship which bind form to form, can- 

 not do more than try to make sure that the pc.-u 

 liarities on account of which he gives a new name 

 to any group of creatures are greater than those 



which distinguish the members of a family of these, 

 are relatively constant from generation to genera- 

 tion, and are associated with reproductive varia- 

 tions which tenil to restrict the range of mutual 

 fertility to the mem hers of the proposed new species. 

 Unfortunately, however, species often are and 

 sometimes must be established for single speci- 

 mens, without any knowledge of their reproduc- 

 tion and generations, without any statistics of 

 their variations, or careful comparison of these 

 with those of related forms. Where the form in 

 question is conspicuously unique the erection of a 

 new species is of course readily justified. 



History. The history of the biological concep- 

 tion of species can be rapidly sketched. We need 

 not go farther back than John Kay ( 1628-1705), 

 who gave to the term species that meaning which 

 it bore until evolutionary conceptions prevailed. 

 Linmeus adopted the usage defined by Kay, and 

 by introducing the ' binomial nomenclature ' made 

 it more serviceable. He believed in real species, 

 and said : ' Species tot sunt diversie, quot diversas 

 formos ah initio creavit infinitum ens,' though 

 some hesitancy in regard to this is shown when lie 

 elsewhere expresses the idea that all the species of 

 a genus ab initio imam constitiicruitt sjtecieii*. 

 Although Lamarck in 1809 declared species to be 

 artificial conceptions, the Linnean idea prevailed 

 even in Agassiz' Essay on Classification (1859) 

 until Darwin and his fellow-workers modified this 

 among many other conceptions by establishing the 

 doctrine of evolution, ' by which," as Kay Lankester 

 says, ' universal opinion has been brought to the 

 position that species, as well as genera, orders, and 

 classes, are the subjective expressions of a vast 

 ramifying pedigree in which the only objective 

 existences are individuals. ' 



See BIOLOGY, BOTANY, DARWINIAN THBOKT, EVOLO- 

 TION, GENUS, VARIATION, ZOOLOGY. See also Darwin, 

 Origin of Specie* ( 1859) ; Haeckel, Generellt Morphologi* 

 (1866); Naturlithe Sehopfuniwietckielite ( 1868 ; 8th ed. 

 1889; trans. Natural History of Creation, Lond. 1879): 

 Die Kalkichwamme (1872); Spencer, Principle* of 

 Biology (1864-66); Wallace, Darwinian (1889). The 

 species of Logic is originally suggested by Natural 

 Species, and may be defined as a group of individuals 

 agreeing in some common character and known by a 

 common name ; two or more species constituting a genus. 

 The relation of species to the other logical elements of 

 classification are treated at I'ltKiiicAin.F.s and GENERAL- 

 ISATION. The great controversies as to the real existence 

 of species and other universals are sketched at NOIIIM- 

 ALI8M. 



Specific Density, the mass of any given sub- 

 stance contained iii unit volume. On the centi- 

 metre-gram me-second system of physical units, 

 since a cubic centimetre of water at standard 

 temperature and pressure weighs 1 gramme, the 

 density of water = 1, and water is the standard of 

 density ; and the specific density of a l>dy is the 

 number of grammes' mass per cubic centimetre. 

 Since, according to the law of gravity, weight* 

 are proportional to masses, ii is convenient to 

 ascertain specific densities by ascertaining the 

 specific gravities of the sulwtances tested. For 

 example, an English gallon of water weighs at 

 standard temperature and pressure (62 F. and 

 barometer 30 inches) 10 Ib. avoirdupois; a gallon 

 of ether weighs 7 -2 Ib. ; the specific density of ether 

 is therefore 7 "2 -4- 10 = 0'72. Similarly, a gallon of 

 strong sulphuric acid weighs 18'4 Ib., and the 

 specific density of sulphuric acid is 1 'S4. The 

 specific densities of solids may be determined by 

 the hydrostatic balance (see ARCHIMKDK.S, PRIN- 

 CIPLE" OF), which gives the weight of a quantity 

 of water equal to that of the solid ; or by using a 

 'specific gravity flask.' This is a flask marked 

 distinctively at a certain level ; the solid is put 

 into this the flask is filled with water up to the 



