September io, 1903] 



NATURE 



457 



for existence on the coral reef. But in the case of the 

 Pennatulid, the natural or normal shape is adapted to a 

 less variable series of environmental conditions, and it has 

 sufticient power of movement to shift itself into localities 

 where the environment is suitable for it. In other words, 

 the power of movement is associated with a loss of plasticity 

 of the character " shape." 



But the growth of corals may be affected in other ways. 

 A great many of these forms of life harbour a small fauna 

 of epizoic Crustacea, mollusra, and worms, and the ramifi- 

 cation or surface is often affected by these in a remarkable 

 way. I have elsewhere pointed out that the character of 

 certain specimens of Millepora, which is known as verrucose, 

 is due to a modification of the growth round epizoic 

 barnacles. Semper has shown that the curious cage-like 

 growths seen on the branches of Seriatopora and Pocillopora 

 are galls produced by the action of certain species of crabs. 

 In a recent paper I have also given reasons for believing 

 that the tubular character of the stem and some of the 

 branches of the genus Solenocaulon is due to the action of 

 certain Crustacea belonging to the family Alpheidae, and 

 that when these Alpheids are not present the form with a 

 solid stem hitherto known as the genus Leucoella is pro- 

 duced. 



But whilst some genera of corals and Alcyonaria are 

 plastic in this way, others are not. These coral galls may 

 be found on the Milleporas and Madreporas of a certain 

 portion of a reef and be absent from all the other genera 

 of neighbouring corals. The crab-galls that are found so 

 commonly and in such abundance upon Pocilloporas and 

 Seriatoporas in certain parts of the Pacific and elsewhere 

 are found only in cases of extreme rarity in other corals. 



Many other cases could be given to show that in some 

 genera the coenenchym is remarkably plastic or accom- 

 modating to these epizoites, whereas in others it is resistent 

 and rigid. 



The size and shape of the spicules have been taken as 

 rharacters for the determination of the species of Alcyonaria. 

 It is true that in some species the spicules are remarkably 

 onstant in size and shape, but in others they are extremely 

 variable. The remarkable torch-like spicules of the 

 oenenchym of Eunicclla papulosa, the club-shaped spicules 

 of Acrophytum, and the needle-shaped spicules of many 

 species of Pennatulids are remarkably constant in size and 

 shape, but in Sarcophytum, the new genus Sclerophytum, 

 Siphonogorgia, Spongodes, and a great many others, the 

 size and shape of the spicules are extraordinarily variable. 

 In the matter of colour, too, we find the same thing. The 

 genera Tubipora and Heliopora are widely distributed in 

 the shallow waters of the tropical seas and are very variable 

 in many of their characters, and yet there is not a single 

 specimen of Tubipora known that is not red, nor a single 

 sjjecimen of Heliopora that is not blue. The same may be 

 saM for several other species. On the other hand, many 

 species of Alcyonaria are extremely variable in colour. 

 I hus, Muricea chamaeleon is, according to Von Koch, 

 onetimes yellow, sometimes red, and in some cases speci- 

 iicns show both red and yellow branches. The specimens 

 f Melitodes dichotoma in Cape waters are sometimes red 

 ind sometimes yellow. In a small species of Melitodes 

 from the Maldive Archipelago there is a very remarkable 

 degree of variation in colour both in the nodes and inter- 

 nodes, the details of which I have briefly described in vol. 

 ii. of Mr. Gardiner's Results. In the genus Chironephthya, 

 also from the same Archipelago, the variations in colour are 

 very remarkable, the spicules of the general coenenchym 

 showing various shades of red, pink, yellow, and orange, 

 and the crown and points purple, yellow, and orange colours 

 which sometimes agree, but usually do not agree, with the 

 general colour of the coenenchym. The variability of the 

 genus is particularly interesting, as in Siphonogorgia, the 

 genus which comes nearest to it, and is, in fact, difficult 

 to separate from it, the colour of the coenenchym is almost 

 invariably red. 



To summarise this knowledge of variability in the 

 Ccelenterata we may say that we find either extreme 

 plasticity or remarkable rigidity in many of their most 

 important characters. Such important and essential organs 

 as the tentacles, stomod,-Eum, mesenteries, &c., are in some 

 groups very variable indeed, and in others as stationary or 

 fixed ; we find the same with organs such as the spicules 



of Alcyonaria, which are, so far as we can judge, of less 

 essential importance, and in characters, such as colour, 

 which must be, in the sedentary forms at least, of minor 

 importance. 



If we compare this with what we find in the higher 

 groups of animals we observe a great contrast. In fishes, 

 to take an example at random, we may find that in such 

 characters as the size and weight of the adults, there may 

 be great or considerable variability, but in the essential 

 organs, such as the heart, brain, and stomach, there is 

 almost complete rigidity. I do not mean by using the 

 expression " rigidity " to imply that minor variations in 

 sizo and shape do not occur, but that major variations, 

 such as a doubling of the stomach, a bifurcation of the 

 cerebral hemispheres or other variations, which it would 

 be considered grotesque to suggest even, do not and cannot 

 occur. But even in minor characters, such as colour, the 

 possible range of variation in a fish is far less than in 

 Coelenterates. We may find in the mackerel, for example, 

 that individuals differ in the shade and range of the green 

 pigment, but we do not find in any species of fish that 

 some individuals are red, some yellow, some purple, &c. 



The contrast in this respect between the Ccelenterate and 

 the fish must be associated with their different degree of 

 complexity of structure. In a complicated organisation 

 such as that of a fish, the brain, heart, and stomach must 

 mutually work together ; they must be co-ordinated in form 

 and action. Any profound variation or abnormality of one 

 would interfere with the action of the others and would 

 therefore be incompatible with continued existence. In the 

 Ccelenterate, however, the doubling of the siphonoglyph, the 

 duplication or quadruplication of the mesenteries does not, 

 in some cases, interfere materially with the action of the 

 other organs of the body. If we were to alter the size or 

 shape of some part of a simple machine it might be able 

 still to do its work the better or the worse for the change, 

 but if we were to alter the corresponding part of a com- 

 plicated machine it would probably throw it out of gear 

 and prevent any work being done at all. 



From this consideration we gather that in the process 

 of the evolution of the higher forms of life there has been 

 a gradual diminution in the range of variation of the 

 different characters of the body, a gradual diminution of 

 the response of these characters to changes of the environ- 

 ment. Characters which, in the early stages of evolution, 

 were probably plastic become rigid. 



The gradual evolution of the power of co-ordinated move- 

 nient has been undoubtedly accompanied by a loss in the 

 variability of the shape of the body, the gradual evolution 

 of a blood vascular system and nervous system has led to 

 a loss of variability in the alimentary canal with which they 

 are associated. In the majority of cases, however, we are 

 much too ignorant of the facts of the co-ordination of the 

 parts of the body or of the co-ordination of any one part to 

 the environment to be able to frame an hypothesis as to 

 why any one character has become rigid, it is difficult to 

 see the reason why the number of the tentacles and mesen- 

 teries in Alcyonian polyps has become fixed at eight, while 

 in other Coelenterates these characters are so variable, or 

 why the colour of Tubipora is always red, and of Melitodes 

 variable. 



The study of species, however, teaches us that, in all 

 cases, except perhaps in some examples of degeneration, 

 the plastic condition of the characters was antecedent to 

 the rigid, that in the earlier stages of evolution the con- 

 dition of extreme plasticity and ready response to changing 

 external conditions were necessary for the survival of the 

 species ; and that in the later stages, when special adapta- 

 tions to special circumstances were developed, a certain 

 rigidity or indifference to changing external conditions was 

 equally necessary for its survival. 



Now, the study of the various orders of Coelenterates 

 conveys a very strong impression that the part played by 

 the environment in the production of the variations of the 

 adult is much greater in proportion than it is in the higher 

 groups of animals. It is true that direct proof of this is 

 wanting. Such a direct proof can only be obtained by 

 experiments in rearing and breeding under varying con- 

 ditions, and there are at present many serious difficulties 

 to overcome before experiments of this nature can be satis- 

 factorily made. 



NO. 1767, VOL. 68] 



