ORGANIC FORM AND ARCHITECTURE 711 



like, of course, to examine both sexes of his new species, and a 

 series showing juvenile, adolescent, mature, and senescent phases, 

 but it is often very difficult to fulfil these requirements, e.g. when 

 one is describing a collection of novelties from the Deep Sea. 



A third consideration, but also difficult to apply, has to do with 

 fertility and sterility. When two related species have been defined 

 in a manner quite satisfactory to the sceptical inquirer it should be 

 possible to say that they do not readily cross and yield fertile off- 

 spring. Apple and pear are two species not very far apart, but the 

 pollen of the one is useless for the other. Rabbits and hares, which 

 are much more distantly related, never have hybrid offspring. It 

 is doubtful if they ever try to cross. And although there are some 

 exceptions, such as wolves and jackals, mallard and pintail, it is an 

 important criterion that while members of a species are usually 

 fertile inter se, they are rarely fertile with other species. But the 

 difficulty is to apply the criterion. Moreover, there are some cases 

 where varieties or races that are usually referred to the same species 

 will not have anything to do with one another as mates. Such facts 

 point to the reasonable conclusion that while a species is a group of 

 similar individuals with a certain individuality, sometimes structural, 

 sometimes biochemical, sometimes habitudinal, these indi\adualities 

 of species are very unequal when compared with one another. 



The fourth consideration which should be kept in mind is plasticity 

 or modifiability. Many types of living creatures are constitutionally 

 obstinate, or so well poised, in their activities and in their archi- 

 tecture alike, that they are not much influenced by changes in 

 nurture — that is to say, in environment, food, and habits. But other 

 creatures are individually very plastic, plants more than animals, 

 sedentary animals more than active ones. It is therefore important 

 to ask whether some species-characters and race-characters may not 

 be the results oi similar environmental, nutritional, and habitudinal 

 dints, which are hammered afresh on to each successive generation. 

 Two nearly related species may be inherently much less different 

 than they seem, for each, with its own peculiarities of nurture, may 

 be bearing the imprint of individually acquired modifications. If 

 this be so, the fourth criterion of a good species must be experi- 

 mental. Are the species-characters altogether innate, or are some of 

 them similarly impressed on the successive crops of individuals 

 who live in particular niches of environmental and nutritional 

 opportunity ? The application of this test of species is extraordinarily 

 difficult. It is a relief to come back to Plato and recognise in each 

 species a distinct idea. Rut the ideas change in space-time, and we 

 have still to ask : What is a species? 



THE 2V1AKING OF A SPECIES.— A species, as we have explained, 



is a group of similar true-breeding individuals which are themselves 



