74 SECTIONAL ADDRESSES 



its functional value as to project beyond the anterior sulcus into a position 

 of maximum risk and minimum usefulness. Almost immediately after 

 that stage was reached, the genus Mia aster disappeared and was no more 

 seen. 



Here again we are driven to the conclusion that a structure, once 

 initiated with the best intentions in the matter of utility, continued to 

 increase regardless of its own efficiency or the welfare of the organism 

 of which it was a part. 



Lastly in this connection, we can record a feature in the morphogeny 

 of Mia aster that often causes difficulty to stratigraphers. When we 

 consider the two outstanding characters of petal-ornament and labrum 

 together, occasional anomalies appear. It is unsafe to use only a fragment 

 of a test for zoning purposes, because the ambulacral character may be 

 either before or behind its ' time ' in the succession, and the same con- 

 dition may apply to the peristomial features. If, however, we find a 

 test with relatively ' low-zonal ' ambulacral petals and relatively ' high- 

 zonal ' labral structure, stratigraphy will always enable us to prove that 

 the proper chronological place of that specimen is somewhere between 

 the extremes indicated by each character separately. In other words, the 

 whole test shows the correct zonal position of the specimen ; it is the 

 average result of the conjunction of its various parts. In practice, this 

 leads to a very safe, but scarcely mathematical, usage. A practised 

 eye can tell by a general look at a Micraster its correct zonal position, 

 whereas laborious analysis of each character separately often leads to 

 contradictory and confusing results. It is a process closely analogous 

 to our recognition of our human acquaintances : we do not consciously 

 remember (if we ever really knew) every detail of each feature. Even 

 lovers, who may be expected to indulge in fairly close investigation, are 

 not always able to recite a reliable catalogue of the facial peculiarities of 

 the object of their regard. 



The general principle that seems to emerge from this scrutiny of 

 some of the features of Miaaster can be expressed in simple terms. 

 Each character has some measure of independence, and follows a morpho- 

 genetic trend (whether of increment or reduction) irrespective of the other 

 characters and even of its own utility. The organism as a whole seems 

 to hold a kind of balance between the several characters, so that if one 

 of them is precocious another will be backward. 



Although this principle has been illustrated here by reference to the 

 history of a single genus (and, for the sake of clearness, of a few characters 

 only), it can be recognised in most of the groups of organisms of which 

 we have much palasontological knowledge. In effect, it is merely a state- 

 ment of the fact of over-specialisation ; but it seems to explain in some 

 degree the danger inherent in that disease. For if the function of the 

 whole organism is to ensure a balance between independent and perhaps 

 antithetic trends of evolution among its components, specialisation in one 

 respect must inevitably involve reduction in others. There is no evidence 

 to suggest that the structures that become highly specialised are necessarily 

 the most vitally useful (although if they are of that calibre their over- 

 development will soon neutralise their value) ; so that over-emphasis 

 of an unimportant or harmful feature must lead to starvation and reduction 



