144 ANIMAL AGGREGATIONS 



the thirty years' interval shown in Table IV that density of popula- 

 tion had only 0.875 the effect during the 1890's that it had in the 

 i86o's. Here we have evidence that relatively mild crowding affects 

 longevity in men. 



This is to be expected when we consider the relatively greater 

 ease of transmission of contagious diseases in the more crowded 

 areas. Such dangers from the crowd are illustrated in a simplified 

 form by one of the Ophioderma experiments to be reported in full in 

 another connection. In these experiments the survival of 8 isolated 

 brittle starfish, each in a i-liter Erlenmeyer flask, was compared with 

 a group of 8 similar starfish in an 8-liter bottle. Usually the group 

 outlived the isolated individuals; but on one occasion one of the 

 members of the group died soon after the daily inspection and change 

 of water, and so polluted the whole 8 liters that all the remainder 

 were dead on the following morning. When an isolated animal died 

 similarly, the effects of its death could not extend beyond the limits 

 of its single flask. 



When we pass in review the materials presented in the chapters of 

 which this is the third, we find much evidence supporting the gener- 

 ally accepted dictum that crowding is harmful for poorly integrated 

 groups of animals, breeding and hibernation seasons excepted. We 

 have seen that crowding may slow down the rate of growth and may 

 result in dwarfed individuals, that the rate of reproduction may be 

 decreased, and that the death-rate may be greater. These effects 

 have been reported for so many different animals from such a wide 

 range of the animal kingdom that there can be no doubt of their 

 general significance. But this is not the whole story. In many of the 

 experiments to be reported in our next section, we shall find that 

 crowding does not always produce harmful results; and that under 

 many conditions there are distinctly beneficial results, providing the 

 crowding be not too great. When considering these beneficial re- 

 sults, we must, however, always keep in mind the harmful effects of 

 overcrowding. 



