Abstract, -A homolog is a part of the phenotype that is homologous to equivalent parts in other species. A biological homology concept is expected to explain three properties of homologs: I) the conservation of those features that are used to define a homolog, 2) the individualization of the homolog with regard to the rest ofthe body, and 3) the uniqueness of homologs, i.e., their specificity for monophyletic groups. The main obstacle to describing a mechanistic basis for homology is the variability of the developmental pathways of undoubtedly homologous characters. However, not all aspects of the developmental pathway are ofequal importance. The only organizational features of the developmental system that matter are those that have been historically acquired and cause developmental constraints on the further evolutionary modification of the characters. Two main factors contribute to historically acquired developmental constraints: generative rules of pattern formation and ontogenetic networks. In particular, hierarchical and cyclical inductive networks have the required properties to explain homology. How common such networks are is an open empirical question. The development and variation of pectoral fin hooks in blenniid fishes is presented as a model for the study of a simple ontogenetic network.

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Econd for three .different values of x by integrating the directly measured specific heat, and obtains LC for roughly corresponding values of x by a plausible ansatz for the optical data (he notes that the values so obtained are consistent with those measured directly); acknowledging that the errors are considerable. he ~o i n t s out