THE GROWTH OF GROUPS 123 



so it may be said ; but if this is a sound argument we 

 shall never observe the origin of a natural species. It 

 is true that man had a hand in starting the group, but only 

 by bringing together a single pair of sports. We know 

 that a group of pallida is not easily established in nature 

 since the event has not yet been witnessed. But thou- 

 sands of the kind are being born as sports every year, 

 since sixty-two of them were found among less than a 

 quarter of a million of the normal kind ; and time is 

 going on. However unlikely the establishment may be, 

 it must eventually happen, unless of course decemlineata 

 itself becomes extinct or ceases to give off sports. How- 

 ever unlikely any particular combination of events may 

 be, given sufficient time it will occur, provided that it 

 continues to be possible. The possibility in this case 

 was demonstrated. 



If pallida had been found established in nature we 

 should of course have known nothing about it as a sport, 

 it would have been a species. But this is a digression. 



We will now consider the most important part of 

 Tower's researches. The potato beetle lays five batches 

 of eggs at intervals of a few days. The second batch 

 does not begin to develop in the ovary until after the 

 first is laid. The third batch not until the second is laid, 

 and so on. This peculiarity in the life history of the 

 beetles afforded a valuable opportunity which many 

 would have overlooked. Fortunately the opportunity 

 was used with good effect, in the following manner. 

 Certain beetles of the species decemlineata were placed 

 under unusual physical conditions, great heat and dryness, 

 during the time when their first three batches of eggs 

 were being formed. The eggs when laid were set aside 



