Hermann, Pernille, Stephen A. Mitchell, and Jens Peter Schjødt, eds., with Amber J. Rose. 2017. Old Norse Mythology—Comparative Perspectives. Milman Parry Collection of Oral Literature 3. Cambridge, MA: Milman Parry Collection of Oral Literature. http://nrs.harvard.edu/urn-3:hul.ebook:CHS_HermannP_etal_eds.Old_Norse_Mythology.2017.
9. Comparing Balto-Finnic and Nordic Mythologies
Anyone who has read either the Kalevala or the underlying poems from oral tradition will recognize that this definition fits them,  and will recognize many of the characters in Agricola’s lists. Indeed, centuries after Agricola compiled his lists, when (or perhaps before?) Johann Gottfried Herder’s ideas of the nation were beginning to be felt in Finland (Pulkkinen 1999), the term mythology was explicitly used, namely in Christfrid Ganander’s 1789 Mythololgia Fennica (Ganander 1984), which despite its Latin title is a work in Swedish, the language of the kingdom of which Finland had long been a part. As the subtitles of the original make clear, Ganander drew his material from oral traditions, most of which were narrative. According to the title page, his sources comprised: “de äldre finska troll-runor, synnyt, sanat, sadut, arwotuxet &c” (the older Finnish magic songs, origins, words, tales, riddles, etc), and he quotes a certain amount of narrative poetry. In his preface, Ganander draws frequent comparisons with classical mythology, but he does profess an interest in two other mythologies from within the kingdom, the Sámi and the Swedish. Indeed, insofar as he promises comparison among these three mythologies, Ganander is the first scholar to attempt a comparison of Balto-Finnic and Scandinavian materials, although his actual comparisons in the alphabetical listings within the Mythologia deal either with earlier scholars’ attempts to create Swedish prehistory out of Old Norse and classical writings, or the so-called “lower mythology”, or “folk belief” of the time.
Itse laulan, | millon kuulen
´ ´ ´ ´
kuta kuulen, | niin kujerran (sung by Arhippa Perttunen; collected by Elias Lönnrot in 1834 in Latvajärvi in the parish of Vuokkiniemi, Archangel Karelia)
(Myself I sing, when I hear;
what I hear, I coo (text and translation in Finnish Folk Poetry p. 81))
´ ´ ´ ´
Hljóðs bið ec allar | helgar kindir
´ ´ ´ ´
meiri oc minni, | mǫgo Heimdalar (Vǫluspá st. 1)
(I ask for a hearing from all the holy peoples,
greater and lesser, the sons of Heimdallr.) 
There are similarities beyond the two stresses per half line: in Balto-Finnic, extra syllables can accrue at the beginning of the a-lines, as in fornyrðislag and West Germanic alliterative poetry; there is a tendency toward variation, as in West Germanic alliterative poetry; and alliteration is very common, although it does not play a structural role, as it does in Germanic poetry. Add all these, and the similarities are certainly thought-provoking. I am not making any argument here about mutual influence or even areal stylistics; but I do think persons from one tradition area could probably hear familiar features in the poetry of the other area, even if they could not understand a word. Probably the initial word stress in both language families caused some of these similarities, but this is not the place to engage in comparative metrics.
Castrén then documents the correspondences between Lemminkäinen and Baldr, both killed by a blind man with a strange weapon:
And there was more. Not content to expose the similarities of individuals, Castrén expanded his gaze to the general populations of the two mythological systems.
Castrén thought that the occupants of Pohjola look like the jǫtnar “giants” of Old Norse, and that Louhi seems reminiscent of Loki, both with respect to her enmity toward the Æsir (that is indeed how he puts it, not the people of Kalevala) and the similarity of name forms. That horse has had long legs, and one still sometimes sees Louhi turning up in the debate on Loki.
Comparison is still a slippery path that can easily lead us astray. But the path is clearer now that Old Norse mythology is closer to having been considered properly and the Finnish material has been more thoroughly collected.