Ixxiv REPORT — 1866. 



In the numerous localities of the Amazon region certain gregarious species 

 of Butterfly (Heliconidea) swarm in incredible numbers, almost outnumbering 

 aU the other butterflies in the neighbourhood ; the species in the diff'erent 

 locahties being different, though often to be distinguished by a very slight 

 shade. 



In these swarms are to be found, in small numbers, other species of butter- 

 flies belonging to as many as ten diflferent genera, and even some moths ; 

 and these intruders, though they structurally differ in toto from the swarms 

 they mingle with, and from one another, mimic the HeHconideae so closely in 

 colours, habits, mode of flight, &c., that it is almost impossible to distinguish the 

 intruders from those they mingle with. The obvious benefit of this mimicry 

 is safety, the intruders hence escaping detection by predatory animals. 



Mr. Bates has extended his observations to the habits of life, food, varia- 

 tions, and geographical range of the species concerned in these mimetic phe- 

 nomena, and finds in every case corroborative evidence of every variety and 

 species being derivative, the species being modified from place to place to suit 

 the peculiar form of Heliconidea stationed there. 



Mr. Wallace has done similar ser\dce to the derivative theory by his obser- 

 vations and writings on the Butterflies and Birds of the Malay Archipelago, 

 adducing instances of mimetic resemblances strictly analogous to the above ; 

 and adding in further illustration a beautiful series of instances where the 

 form of the wing of the same butterfly is so modified in various islets as to 

 produce changes in their mode of flight that tend to the conservation of the 

 variety by aiding its escape when chased by birds or predacious insects. 



He has also adduced a multitude of examples of geographical and repre- 

 sentative species, races, and varieties, forming so graduated a series as to 

 render it obvious that they have had a common origin. 



The effect of food in the formation and segregation of races and of certain 

 groups of insects has been admirably demonstrated by Mr. B. D. Walsh, of 

 North America. 



Dr. M'^Donnell has been led to the discovery of a new organ in electric 

 fishes from the application of the theory of descent, and Dr. Fritz Miiller 

 has published numerous observations showing that organs of veiy different 

 structure may, through the operation of natural selection, acquire very 

 similar and even identical fimctions. Sir John Lubbock's diving hymeno- 

 pterous insect affords a remarkable illustration of analogous phenomena ; it 

 dives by the aid of its wings, and is the only insect of the vast order it belongs 

 to that is at all aquatic. 



The discovery of the Eozoon is of the highest importance in reference to 

 the derivative hypothesis, occurring as it does in strata that were formed 

 at a period inconceivably antecedent to the presupposed introduction of life 

 upon the globe, and displacing the argument derived from the supposition 

 that at the dawn of life a multitude of beings of high organization were 

 simultaneously developed (in the Silurian and Cambrian strata). 



Professor A. De CandoUe, one of the most distinguished continental bota- 

 nists, has, to some extent, abandoned the tenets held in his ' Geographic 

 Botanique,' and favours the derivative hypothesis in his paper on the varia- 

 tion of oaks ; following up a paper, by Dr. Hooker, on the oaks of Palestine, 

 showing that some sixteen of them are derivative, he avows his belief that 

 two-thirds of the 300 species of this genus, which he himself describes, are 

 provisional only. 



Dr. Hooker, who had only partially accepted the derivative hypothesis 

 propounded before the publication of ' The Origin of Species through Natural 



