ferences in populations not yet genetically analyzed.(see especially dobzhansky,1951;and on application of some of the principles to fossil populations,see,for example,simpson,1953)Behavioral data have frequently been viewed with suspicion by taxonomist for the same reasons previously mentioned for physiological data.Indeed first level or elemental behavior,involving suchtings as posture and geometrically possible limb movements,is physiological and does suffer from the same taxonomic problem as the simpler,more generalized physilogical traits:it is particularly subject to convergence.That problem is however,much less serious for second level or compound behavior,involving more or less complicated patterns or programs compounding the elemental movements,as for example,in bird courtship,Mayr(1958,several other chapters in Roe and Simpson,1958 also have some relevance here)Has semonstrated and adequately exemplified the taxonomic userfulness of behavior at this level,where it is relational rather than individual. Finnaly,more explicit recognition of the sample-Population relationship has involved another marked change in the collection of the taxonomic data.It was,of course,recognized even in the antiquity that many animal species include sharply different forms such as,for instance,males and females or larvae and imagos.It is also an old discovery that many species are polymorphic in a stricter sense,with distinct forms at one life stage and in one sex.The need for including such different forms was rather widely,although not consistently,recognized by pre-evolutionary taxonomists.Until comparatively recently it was,however,the general opinion(with the usual exceptions) that data from one spiceman of each form,or one for the whole species if there were no obviously different forms,was sufficient for taxonomic purposes.Other specimens,if collected at all,were"duplicates"and were commonly traded to other museums or collectors.Modern taxonomic practice,concerned with populations and not types,requires as far as possible series of specimens large enough for inference as to the total variation in the population from which the sample is drawn.Statistical methods provide precise criteria for judgment of the adequacy of a sample in hand for the inferences made from it,and also for the sizes of samples necessary to draw inferences with a given level of probabilty,or rather of confidence,in particular instances.Of course it is not always practical to obtain a sample of optimum size.That is likely to be praticularly true of fossils,even though some very large samples are now available,especially for invertbrates*).Small samples and single specimens are not useless,however.They only give lower confidence levels and poorer estimates of population ranges,and these deficiencies can and must be taken into account.(See simpson,Roe,And Lewontin,1960).Although the inadequacy can be overcome in time,inadequate sampling is also still a serious problem as regrads some neontological data.For example,interfamilial and interordinal comparisons in systematic serology still rest in considerable part on single individuals of single species in the higher taxa compared,and the results must be correspondingly insecure as bases for taxonomic inferences.
SIMILARITY AND HOMOLOGY
as has already been mentioned and will be further discussed,evolutionary taxonomy does not rely solely on associations of similarity or in principle define all taxa by characters in common.The observation and interpretationof characters in common do nevertheless play a large and essential part in evolutionary taxonomy,as they must in most systems of classification.They have other roles as well,but much of their importance is that they are one,and on the whole the cheif,of the several criteria for judging propinquity of descent(Darwin's apt pharse).Propinquity of descent,in turn,is an important one of the several criteria for allocating ranks to taxa and corresponding taxonomic priorities to characteristics of organisms.Similarities are most directly indicative of propinquity of descent if they earlier occured in a single taxon ancestral to the later taxa in some or all of which the similarities are retained,in other words,if they have been inherited from a common ancestry.As was already well known to Darwin and has become still more evident since,this is not the only way in which similarities arise in the course of evolution.Although absent in the common ancestry,they may arise in some or all descendant taxa as parallel developments channeled by characteristics,genetical or other,of the ancestry.In that case they still have bearing on propinquity of descent,but the bearing is less direct and may be less definitive.They may and frequently do also arise as independent convergent adaptations to similar ways of life in taxa of quite different ancestries.In that case they have no bearing on propinquity of descent and are a major potential source of confusion or error in evolutionary taxonomy.**)Similarities can also arise from mimicry,in which the similarity is itself the pertinent adaption and from chance,in which the similarities have separate and unrelated causes.These,too,have no bearing on propinquity of descent,but in them the similarities are so superficial that they are rarely confusing.One of the really basic problems of evolutionary taxonomy is to distinguish among these various kinds of similarities and particularly between those that are and those that are not inheretied from a common ancestry.For purposes of discussing them a vocabulary is needed and the following definitions are here adopted:homology is resemblance due to inheritance from a common ancestry.The similar characters involved are homologus,and the noun for them is homologues.homology is resemblance due to inheritance from a common ancestry.The similar characters involved are homoplastic.There is no current noun for them.Homoplasy includes parallelism,convergence,analogy,mimicry,and chance similarity.Parallelism is the development of similar characters separately in two or more lineages of common ancestry and on the basis of,or channeled by,Characteristics of that ancestry.Characters so developed are parallel,but again there is no current noun for them.convergence is the development of similar characters separatly in two or more lineages without a common ancestry pertinento the similarity but involving adaptation to similar ecological status.Similarities so developed are convergent.(Again no noun)Analogy is functional similarity not related to community of ancestry.The characters involved are analogues.and the noun for them is analogues.Convergent characters are analogous insofar as the similarity can be related to function,which is ussually and perhaps always the case mimicry is similarty adaptive as such as and not related to community of descent.It occurs when one group of organisms resembles another of different descent within the same community and when the resemblance is adaptively advantageous to the mimicking organism for any of several reasons,specification of which need not concern us here.The mimicking organism is the mimic and that mimicked is its model.There are no commonly used special terms for the characteristics involved.
Chance similarity is resemblance in characteristics developed in separte taxa by independent causes and without causal relationship involving the similarity as such
*)I Doubt whether finances,storage space,and time or,indeed,availability in the field will ever permit gathering entirelly adequate samples of species of sauropod dinosaurs ,for example.?
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