Vol 50, No 6 (2019)

The 160 years that have passed since the release of the first edition of the brilliant work by Charles Darwin On the Origin of Species through Natural Selection were extremely fruitful for biology, primarily for the development of the theory of evolution and species concept. Despite this, almost all problems formulated by Charles Darwin are still relevant, including the problem of the species and the role of hybridization in evolution.

Since Charles Darwin (1809–1882) and Ernst Haeckel (1834–1919) published their trailblazing ideas, the scientific community’s discussion of evolutionary biology has included the topic of embryological development. The concepts of ontogeny and phylogeny, still current in contemporary biology, together with the now obsolete biogenetic law and his Gastraea theory, which trace back to Haeckel, all underwent an evolution of their own in Haeckel’s works. The record of this evolution makes clear how the features of his thinking that proved durable, such as ontogeny and phylogeny, were established as such through a difficult creative process of formation of concepts, theories, and terminology that themselves enjoyed varying fortunes. Beginning with Haeckel’s Generelle Morphologie der Organismen [General Morphology of Organisms] (1866), this paper traces aspects of the conceptual and terminological evolution that takes place both within the pages of this highly complex but seminal work and then chronologically in later works. We include the use of text data mining of his works to establish and analyse word frequency patterns. We seek to indicate here some of the challenges Haeckel faced in establishing new concepts and terminology in the General Morphology (hereafter GM), and we draw attention to his efforts in later works to extend this didactic work.

The problem of the origin of the metazoan life cycle is analyzed on the base of various hypotheses on the origin of multicellular animals. According to the Gastraea theory and the Phagocytella theory, the ancestral metazoan life cycle was holopelagic. In the framework of the hypotheses of the primary sedentarity, the metazoan ancestor had a pelago-benthic life cycle with floating larvae, the synzoospores. In accordance with this hypothesis, Eumetazoa originated from the progenetic larvae of the sedentary ancestor. The primary life cycle of Eumetazoa (i.e., metazoans with a nervous system, musculature, mouth, and gut) was holopelagic. Particularly this life cycle is typical for recent Ctenophora, which is the earliest branch of the eumetazoans. Cnidaria and Bilateria are sister groups. Their last common ancestor acquired the pelago-benthic life cycle de novo. The pelagic part of the life cycle of cnidarians is comprised of the blastula and gastrula stages only. Some anthozoans still maintain the planktotrophic gastrula larvae in their life cycles. Planulae of Medusozoa are simplified lecithotrophic larvae that had lost the function of spread because of the appearance of the medusa stage in the life cycle. In triploblastic Bilateria, the prolongation of the pelagic stage of the life cycle occurred due to the appearance of the ciliated bilaterally symmetrical larvae, which are actually the juveniles raised into the water column. This phylogenetic modus can be designated by the special term “larvalization.” Thus, the ciliated pelagic larvae of all bilaterians have a common origin from the juvenile stages of the last common bilaterian ancestor. Their ciliated bands came from the modified ciliated tentacular apparatus of juvenile stages of the last common bilaterian ancestor. Homologous elements in the ciliated bands of trochozoan and deuterostomian larvae are traced.

In most angiosperms, floral organs are acropetally initiated, i.e., from the perianth upwards to the gynoecium. The review surveys examples of deviation from this typical pattern that takes place in both oligomerous and polymerous whorled flowers. The plants displaying the same non-acropetal pattern of floral development are not necessarily closely related and thus similarities in their floral structure and development should be regarded as convergences. Vice versa, representatives of the same family often show different patterns of initiation of floral organs. Flowers with the same groundplan can demonstrate either typical acropetal or non-acropetal developmental pattern. In other words, evolution of patterns of floral development is relatively homoplastic. Presumably, the repeated transitions from acropetal to non-acropetal developmental patterns (and back) readily occurred in evolution and were of a saltational nature.