THE PRESIDENTIAL ADDRESS 17 



a branch of its food-plant fixed over a wren's nest containing young, 

 were unnoticed by the parent bird which used the same branch as 

 a convenient perch ; yet seen and at once taken when placed on a 

 white surface below. 



One of the best examples of a prophetic instinct is to be found in 

 the larva of an African Tabanid fly (T. biguttatiis) . This maggot 

 lives and feeds in soft mud which, during the dry season when the 

 chrysalis stage has been reached, will be traversed in all directions 

 by wide and deep cracks in which insectivorous animals can search 

 for prey. But the maggot, while the mud is still soft, prepares for 

 this danger. By tunnelling spirally up and down it makes a line of 

 weakness which will cause a pillar to separate from the mass when 

 the mud hardens and contracts. It then tunnels into the still soft 

 pillar and becomes a chrysalis in the centre of its deeper end. How- 

 ever wide the cracks which appear in the mud, the maggot has 

 arranged beforehand that they will not invade its cylinder. Dr. 

 W. A. Lambom, who made this most interesting discovery, observed 

 that the summits of the pillars, forming circular discs of about the 

 size of a penny, scattered here and there over the surface, were never 

 thus traversed, but that an empty shell was protruding from the 

 centre of each when the fly had emerged. ^^ My friend the late Prof. 

 J. M. Baldwin, the distinguished American psychologist, well 

 remembered at many of our meetings, wrote when he heard of this 

 discovery : ' it seems complete — one of those rare cases of a single 

 experience being sufficient to establish both a fact and a reason for 

 the fact ! It is beautiful.' 



I would ask any believer in Lamarckian evolution, or in Hering's 

 and Samuel Butler's theory of unconscious memory residing in the 

 germ-cells, how it would be possible to explain these prophetic 

 instincts, adapted not to meet but to avoid future experience, except 

 by the operation of natural selection. 



The appeal to Orthogenesis, or internal developmental force, as 

 the motive cause of evolutionary progress has often been made — 

 generally by palaeontologists rather than by the observers of living 

 forms. Any such belief in the potency of an internal tendency is, 

 I think, open to the criticism made by Thiselton Dyer in his address 

 to Section D at Bath in 1888 : ' This appears to me much as if we 

 explained the movement of a train from London to Bath by attri- 

 buting to it a tendency to locomotion. Mr. Darwin lifted the 

 whole matter out of the field of mere transcendental speculation by 

 the theory of natural selection, a perfectly intelligible mechanism 

 by which the result might be brought about. Science will always 



1* Proc. Roy. Soc, B, vol. 106, 1930, p. 83, pi. v ; Proc. Ent. Soc. Lond., vol. v, 

 1930, p. 14. 



