244 THEORETICAL BIOLOGY 



THE PICTURE OF THE SPECIES 



If some consider insufficient the picture of the species 

 that I have briefly sketched above, let them try the following 

 method, in order to get it clearer. The cinematograph enables 

 us to consider an animal as a continuous series, and thus to 

 pay special attention to each one of its actions. 



Such serial pictures of animals assist our concrete view 

 of things very greatly. We can use them to make a picture 

 of the species. Let us take a very common animal, familiar 

 to us, and one whose habits we know very well : a cabbage- 

 white butterfly will serve. Imagine that all the eggs, which 

 diverge from one another in their genotype, are collected 

 together on a circular surface. They all begin to develop 

 at the same time. Pile up the developmental stages one on 

 top of the other like a rouleau of coins, so that together they 

 form an upwardly growing stem. As soon as the phenotype 

 is ready and the caterpillars hatch out, let them all crawl 

 away from one another in every direction. Now note the 

 fate of the various caterpillars with reference to their different 

 properties. Some find the food that suits them on the cabbage 

 leaves, with which they are very closely inter-adjusted. Some 

 of the others succumb before their numerous enemies in the 

 shape of mites, ichneumon-flies or birds. The inter-adjustment 

 of the caterpillars vis-a-vis of these enemies is more or less 

 adequate according to their properties. Consequently many 

 perish. Finally all the survivors pupate at the same time, 

 and hang in a circle around the original stem, like regularly 

 arranged berries. Again the stages in the metamorphosis 

 are noted in an upward direction, until the butterflies rise 

 like a white cloud and pass in towards the centre, where 

 they unite in pairs and lay their eggs on a circular surface, 

 after which they fall down like withered leaves. From the 

 eggs grows up the new stem. 



