168 INSECTS AND HEREDITY 



the fact that insects as food are of far greater importance 

 than might be at first sight supposed ; for they supply 

 not only the insectivorous species but those other forms 

 which in turn prey upon them. 



Thus, when we bring together the evidence supplied 

 by the study of insects it is seen that it nowhere sup- 

 ports the assumption upon which Lamarckian evolution is 

 founded, the assumption that acquired characters are 

 transmissible by heredity. 



Before leaving the Chair at the conclusion of my second 

 year of office I desire warmly to thank the Officers, 

 Members of Council, and Fellows of the Society, who by 

 their kindness have made my task so easy and altogether 

 pleasurable. You will, I know full well, accord the 

 same generous sympathy to my successor, and under 

 his guidance I feel confident that the prosperity of 

 recent years will be continued, I hope in even larger 

 measure. 



Before taking leave of the Fellows in my official 

 capacity I desire to direct their attention to two thoughts, 



insect-eating birds of one generation is on the average replaced in the 

 next generation by a pair and only a pair of its offspring. Consequently, 

 each pair produces relatively enormous numbers of young which are 

 destined to destruction. If, merely as an illustration, we suppose that the 

 duration of the natural individual life in a given species of small bird is 

 ten years, and that five young are produced each year, then the pressure 

 upon insect life of a single family during one generation is that exerted 

 by two adult birds, the parents, by the two offspring which will survive 

 and take their place, and by forty-eight offspring which will themselves 

 succumb in the struggle for existence at various ages. Special attention 

 is here directed to the pressure of these forty-eight, of which a large 

 proportion will not survive for more than a very small fraction of the 

 natural term of life. Every season will bring forth its fresh supply of 

 five young uneducated enemies of insects. Any of these which live long 

 enough to feed themselves will enter upon an educational career during 

 which they will exert that very kind of selection which tends towards 

 Mullerian (Synaposematic) mimicry. After exerting this pressure for 

 a variable time they will themselves succumb ; and there is no doubt that 

 only a very small proportion will survive until their education is complete. 

 Extending these considerations to the whole community of insect-eating 

 birds in any country, we realize at once that the selective pressure of the 

 inexperienced enemies is, as has been maintained above, many times as 

 great as that exerted by the experienced and educated. 



