5 z6 



THE QUARTERLY REVIEW OF BIOLOGY 





stage is entirely devoted to the preparation 

 and the dissemination of the eggs. This 

 is the case with the magnificent great 

 moths of the family Saturnidas, and also 

 with various other insects in which in 

 the adult stage the mouth parts are 

 defective. In the majority of the flying 

 insects most of the feeding is done in the 

 wingless stages, the flying stage being 

 devoted especially to propagation. 



GROWTH AS A CHANGE OP TYPE 



The growth of animals from the young 

 to the adult stage is not a simple matter of 

 increase in size, for with increasing size 

 there is a corresponding change in all the 

 relations to environment. Often this 

 change is slight, as in dogs, cats and cattle, 

 but more often it is very marked, as in the 

 insects and in the sea invertebrates. 



Most animals, therefore, must be con- 

 sidered not as a single type of creature of 

 increasing size, but as a series of different 

 and successive types each with its special 

 range of limiting conditions, and each 

 with its peculiar problems in securing food 

 and in avoiding enemies which it must 

 overcome in order to exist. 



As an illustration I may mention that 

 pretty little butterfly known in England 

 as the large blue (Lycana arion). On 

 hatching from the egg the tiny caterpillars 

 of this butterfly feed on the flower heads 

 of the thyme. They are protected from 

 their enemies by ants which constantly 

 attend them, attracted by a honey-like 

 secretion they produce. They are in- 

 veterate cannibals, and if two meet one 

 always eats the other. Later they leave 

 the thyme and, crawling down into the 

 ants' nest, live wholly as carnivores 

 preying on the larvae of the ants. They 

 are now no longer cannibals. They trans- 

 form to pupse in the ants' nest, and the 

 adult butterflies crawl out through the 

 ant passages. 



This insect is first a vegetarian cater- 

 pillar living in the open, later a preda- 

 ceous grub living under ground, and 

 finally a butterfly. These three quite 

 different forms require three quite different 

 sets of internal and external chemical 

 and physical adjustments. 



The question of survival in each animal 

 species is in the majority of types asso- 

 ciated with the passage of the individual 

 through more or less widely different 

 types or forms as illustrated by this butter- 

 fly and, strange as it may seem, except for 

 mammals, the dominant animals of the 

 present day, both on the land and in the 

 sea, are those in which there is the maxi- 

 mum diversity between the younger and 

 the adult stages. 



VARIATION 



How is it possible for a single individual 

 in its development to pass from one form 

 into another wholly different in both 

 internal and external chemical and physi- 

 cal relationships? And how is it possible 

 for closely related types, like the large 

 blue of England and our common blues, 

 to have life histories which are almost 

 wholly different? 



The answer to this question is to be 

 found in a study of variation. Every 

 animal type at every stage varies in all 

 directions from the normal form. The 

 limit of variation as we see it is determined 

 solely by the ability of the variants to 

 survive under the conditions by which 

 they are surrounded. Extreme variants 

 are usually classed as aberrations, abnor- 

 malities or deformities, but in reality 

 there are no such things in nature. 



In our contemplation of the animals 

 we are prone to regard any given indi- 

 vidual as normal if it is similar to the 

 majority of others of the same restricted 

 species, but if it departs more or less 

 widely from the form which we regard 





