January 28, 1909] 



NA TURE 



385 



The same law of increase of numbers in proportion to 

 areas applies to the animal world, if we count all the 

 species that visit a garden or field during the year, though 

 those that can continuously live there are not perhaps so 

 numerous in very small areas. 



The Increase of Plaiils and Aiiimah. 



The powers of increase of plants and animals were next 

 ciscussed, and were shown to be enormously great. An 

 oak tree may produce some millions of acorns in a good 

 year, but only one of these becomes a tree in several 

 hundred years to replace the parent. Kerner states that a 

 common weed, Sisymbrium Sophia, produces about three- 

 quarters of a million of seeds ; and if all these grew and 

 multiplied for three years, the plants produced would cover 

 I he whole land surface of the globe. 



Equallv striking is the possible increase in the animal 

 world. Darwin calculated that the slowest breeding of 

 all animals, the elephant, would in 750 years, from a single 

 pair, produce nineteen millions. Rabbits, which have 

 several litters a year, would produce a million from a 

 single pair in four or five years, as they have probably 

 done in Australia, where they have become a national 

 calamity. As illustrative of this part of the subject, the 

 lecturer referred at some length to the cases of the bison 

 and the passenger pigeon in North America, and the 

 lemmings of Scandinavia. In the insect tribes still more 

 rapid powers of increase exist. The common flesh-fly goes 

 through its complete transformations from egg to perfect 

 insect in two weeks, and Linnaeus estimated that three of 

 these flies could eat up a dead horse as quickly as a lion. 



It is these enormous powers of rapid increase that have 

 ensured the continuance of the various types of existing life 

 from the earliest geological ages in unbroken succession, 

 while it has also been an important factor in the produc- 

 tion of new forms which have successively occupied every 

 vacant station with specially adapted species. 



Inheritance and Variation. 



The vitally important facts of inheritance with varia- 

 tion were next discussed, and their exact nature and 

 universal application pointed out. The laws of the fre- 

 quency and the amount of variations, and their occurrence 

 in all the various parts and external organs of the higher 

 animals, were illustrated by a series of diagrams. These 

 showed the actual facts of variation in adult animals of 

 the same sex obtained at the same time and place, which 

 had been carefully measured in numbers varying from 

 twenty to several thousand individuals. 



The general result deduced from hundreds of such 

 measurements and comparisons was that the individuals 

 of all species varied around a mean value, that the numbers 

 becaine less and less as we receded from that mean, and 

 that the limit of variation in each direction was soon 

 reached. Thus, when the heights of 2600 men, taken at 

 random, were measured, those about 5 feet 8 inches in 

 lieight were found to be far the most numerous. About 

 half the total number had heights between 5 feet 6 inches 

 and 5 feet 10 inches, while only ten reached 6 feet 6 inches, 

 or were so little as 4 feet 10 inches, and at 6 feet S inches 

 and 4 feet 8 inches there were only one of each. 



The diagrams from the measurements of various species 

 of birds and mammals were shown to agree exactly in 

 general character, and the further fact was exhibited by 

 iill of them that the parts and organs varied more or less 

 independently, so that the wings, tails, toes, or bills of 

 birds were often very long, while the body or some other 

 part was very short, a point of extreme importance, as 

 - iiplying ample materials for adaptation through natural 

 I tion. 



The Law of Natural Selection. 



The next subject discussed was the nature and mode of 

 action of natural selection. It was pointed out that since 

 the Glacial epoch no decided change of species had occurred. 

 This showed us that the adaptation of every existing 

 species to its environment was not only special, but general. 

 The seasons changed from year to year, but the extremes 

 of change only occurred at long intervals, perhaps of many 



NO. 2048, VOL. 79] 



centuries, with lesser, but still very considerable, varia- 

 tions twice or thrice in a century. It was by the action of 

 these seasons of extreme severity at long intervals, whether 

 of arctic winters or summer droughts, that the very e.xist- 

 ence of species was endangered ; and it was at such times 

 that the enormous population of most species and their 

 wide range over whole continents always secured the pre- 

 servation of considerable numbers of the best adapted in 

 the most favoured localities. Then the rapidity of multi- 

 plication came into play, so that in two or three years 

 the population of each species became as great as ever, 

 while, as all the least favourable variations had been 

 destroyed, the species as a whole had become better 

 adapted to its environment than before the almost catas- 

 trophic destruction of such a large proportion of them. 



It is the fact of the adaptation of almost all existing 

 species to a continually fluctuating environment — fluctu- 

 ating between periodical extremes of great severity — that 

 has produced an amount of adaptation that in ordinary 

 seasons is superfluously complete. This is shown by the 

 well-known fact that large numbers of adult animals that 

 have not only reached maturity, but have also produced 

 offspring and' successfully reared them, continue to live 

 and breed for many years in succession, although varying 

 considerably from the mean, while almost the whole of 

 the inexperienced young fall victims to the various causes 

 of destruction that surround them. 



The Nature of .idaplation. 



The next subject discussed was the complex nature of 

 adaptations in many cases, and probably in all, a subject 

 of great extent and difficulty. The lecturer directed special 

 attention to the relations between the superabundance of 

 vegetation in spring and summer, the enormous, but to 

 us mostlv invisible, hosts of the insect tribes which devour 

 this vegetation, and the great multitudes of our smaller 

 birds the young of which are fed almost exclusively on 

 these insects. Without these hosts of insects the birds 

 would soon become e.xtinct, while without the birds the 

 insects would increase so enormously as to destroy a con- 

 siderable amount of vegetable life, which would, in its 

 turn, lead to the destruction of much of the insect, and 

 even of the highest animal groups, leaving the world 

 greatly impoverished in its forms of life. 



The vast numbers of insects required daily and hourly 

 to feed each brood of young birds was next referred to, 

 and the wonderful adaotation of each kind of parent bird 

 which enables it to discover and to capture a sufficient 

 quantity immediately around its nest, in competition with 

 manv others engaged in the same task in every copse and 

 garden, was next pointed out. The facts were shown to 

 involve specialities of structure, agilitv of motions, and 

 acuteness of the senses, which could only have been 

 attained by the preservation of each successive slight varia- 

 tion of a beneficial character throughout geological time ; 

 while the emotions of parental love must also have been 

 continuouslv increased, this being the great motive power 

 of the strenuous activity exhibited by these charming little 

 creatures. 



Lord Salisbury on Natural Selection. 



As illustrating the strange and almost incredible mis- 

 conceptions prevailing as to the mode of action of natural 

 selection, the lecturer quoted the following passage from 

 the late Lord Salisbury's presidential address to the British 

 Association at Oxford in i8q4. After describing how the 

 diverse races of domestic animals have been produced by 

 artificial selection. Lord Salisbury continued thus : — 



" But in natural selection, who is to supply the breeder's 

 place? L^nless the crossing is properly arranged the new 

 breed will never come into being. What is to secure that 

 the two individuals of opposite sexes in the primeval 

 forest, who have been both accidentally blessed with the 

 same advantageous variation, shall meet, and transmit by 

 inheritance that variation to their successors? Unless this 

 step is made good the modification will never get a start : 

 and vet there is nothing to ensure that step but pure 

 chance. The law of chance takes the place of the cattle- 

 breeder or the pigeon-fancier. The biologists do well to 

 ask for an immeasurable expanse of time, if the occasional 



