76 THE ECOLOGY OF ANIMALS 



densities raises the whole question as to what is 

 meant by biological success. Does it mean main- 

 taining the highest density ? 



An illuminating study by Nicholson (1927) shows 

 the need for careful definition of terms when we 

 speak of a new variation being useful to a species. 

 It is normally assumed that such a useful variation 

 will enable the species to increase in competition 

 with other ones. Nicholson points out that if 

 natural selection favours a cryptic (concealing) colour 

 pattern protecting a butterfly from birds the imme- 

 diate result will be the laying of eggs in higher 

 density (since more butterflies escape attack). This 

 results in a higher density of larvae, which in turn 

 enables insect parasitoids to find more larvae in 

 a given time and so produce a higher degree of 

 parasitization. This finally brings down the density 

 of adult butterflies which survive parasite attacks 

 (these being often of the order of 90 per cent, of 

 mortahty causes). It is clear, therefore, that the 

 original natural selection by birds may change the 

 colour pattern, but does not necessarily increase the 

 density of the population. Ecologically, nothing has 

 been gained by the species except the possession of 

 a new cryptic colour pattern. This process has 

 been called ' intra-specific selection ' (Poulton, 1929). 

 In actual fact, Nicholson believes that such inter- 

 relationships wiU cause oscillations in numbers. 



