His father inherited the Bottisham Hall property in 1787, on the death of his cousin Soame Jenyns (1704-1787), he was a magistrate, landowner and a Prebendary of Ely Cathedral. He was the Chairman of the Board of Agriculture in London and Chairman of the Bedford Level Corporation. His mother (as Mary Heberden) had her portrait painted by Gainsborough. Jenyns was related through his mother's family to Dr William Wollaston (1766-1828) the chemist.
Jenyns was born on the 25th May 1800, his earliest memory was of the funeral of Lord Nelson in January 1806. By 1812, encouraged by his great uncle Rev Leonard Chappelow (1744-1820)of Roydon, he took up the study of natural history. He left School at Putney in 1813 for Eton. There he read Gilbert White's "Natural History of Selborne", this he copied out and got most of it by heart, White's lifestyle and methods Jenyns later took for his own. In 1817 Jenyns was introduced to Sir Joseph Banks as "the Eton boy who lit his rooms with gas". He was always methodical, studious and quiet,unwilling to waste time.
He went to St. John's College, Cambridge in 1818. During his second year his interest in natural history came to the attention of John Stevens Henslow (1796-1861). They worked together on natural history pursuits until Henslow's death. Henslow married Jenyns' sister Harriet in 1823. Jenyns took his degree in 1822, became a Fellow of the Linnean Society and of the Cambridge Philosophical Society and with Henslow set up that Societies Museum. In May 1823 he was ordained Deacon by the Bishop of Lincoln (Pelham) at Mary-le-bone Church, London, and licensed to the Curacy of Swaffham Bulbeck in Cambridgeshire. In 1824 he was ordained Priest by the Bishop of Bristol (Kaye) in the Chapel of Christ's College, Cambridge. On the 7th July he was Licensed to the Perpetual Curacy of West Deerham, Norfolk. In 1824 he published a "Scientific Journal Book" for the making of Meteorological and other observations. In 1825 he proceeded to his M.A. degree and read his first paper on the "Ornithology of Cambridgeshire" to the Cambridge Philosophical Society. He was an original member of the "Zoological Society of London". In December 1827 he became the Vicar of Swaffham Bulbeck. He read a paper to the Linnean Society on "Two British Species of Plecotus" (1828). About this time (1829) he declined a Zoological Professorship being contemplated at Cambridge. He published "The Common Bat of Pennant", Linnean Society (1829); "On Incisor Teeth of Rodenta", Loudons Magazine (1829); The Natterjack of Pennant", Cambridge Philosophical Society (1830); "On the Preceding Severe Winter" (1830) and "On a Peculiar Parasitical Mite found on Slugs" (1831) Loudons Magazine; "On the British Species of Cyclas and Pisidium" (1831)and read a series of lectures on Ornithology to the Cambridge Philosophical Society (1831).
His diary entry for 1831 records "This year I had the offer of accompanying Capt. Fitzroy, as Naturalist, in the Beagle, on his voyage to survey the coasts of S.America, afterwards going round the globe:- declined the appointment wh was afterwards given to Charles Darwin Esq. of Xts' College Cambridge".
His reason for refusing was that his Parish work had reached a stage where he could not in all conscience leave it and his health had never been perfect.
He joined the British Association for the Advancement of Science (B.A.A.S.)at the Oxford Meeting of 1832 and in 1833 published an article in Loudon's Magazine on "Genera and Subgenera". He attended the B.A.A.S. at Edinburgh in 1834 and read his "Report on the Progress of Zoology". He became an original member of the Entomological Society in 1834.
He was elected a Member of the Geological Society of London in 1835. He published two books "A Systematic Catalogue of British Vertebrate Animals" and a "Manual of British Vertebrate Animals" (1835)published by the Syndics of the Cambridge University Press, this last was held in high estimation as a work of reference. In 1836 he wrote "Some remarks on the Study of Zoology" in the first volume of "Magazine of Zoology and Botany" (published 1837).
His first memoir on the "Shrews" was published in the Magazine of Zoology and Botany in 1837 and a second in the Annals of Natural History in 1838 and two papers in the same Magazine on "A New Bat found in Durham" and in 1839 "Three Undescribed Species of Cimex". He also contributed three articles to the "Cambridge Portfolio" (1839) and a review of "Etudes de Micromammal par De Selys-Longchamps" (1839)in the Annals and Magazine of Natural History. He was elected an Honorary Member of the Boston Society of Natural History (1839).
At the earnest request of Charles Darwin he undertook to edit the monograph on the "Fishes" for the "Zoology of the Voyage of the Beagle" (1840-1842). He wrote an article on the "Smaller British Mammalia" in the Annals and Magazine of Natural History (1842). He was elected an Honorary Member of the Royal Zoological Society of Ireland (1842). In 1843 he published a new edition of "The Natural History of Selborne", with notes.
He married Jane, daughter of the Rev. Edward Daubeny of Ampney Crucis, Gloucestershire, in 1844, her uncle was Professor Daubeny of Oxford, who Jenyns often visited. Jenyns was one of the original members of the Ray Society (1844).
He read a paper on the "Turf of the Cambridge Fens" to the Natural History Section of the B.A.A.S. at Cambridge (1845). His book "Observations in Natural History" (1846) was based on his lifetimes records of natural history in Cambridgeshire. He also edited the "Naturalists Almanack" for 1847, published by Van Voorst.
In 1849, Jenyns left Swaffham Bulbeck with his wife to travel to Ventnor, Isle of Wight "on account of her ill health" and in 1850 to a house at South Stoke, near Bath. In July 1851 Jenyns officiated at the wedding at Hitcham of his niece to Dr J.D.Hooker of Kew.
He moved to Swainswick, Bath in 1852, and took up duties at Langridge and Woolley Churches. In 1853 his parishioners at Swaffham Bulbeck presented him with a "testimonial" of 49 volumes of Divinity. In 1855 he founded and was President of the Bath Natural History and Antiquarian Field Club. He donated his large collection of insects to the Cambridge Philosophical Society.
Jenyns read a paper on "The Variation of Species" to the Cheltenham meeting of the B.A.A.S. in 1856, a paper that Darwin asked Jenyns to send him. In 1858 he published "Observations in Meteorology" being the results of 19 years of record keeping at Swaffham Bulbeck. He published a short paper "Note on the smaller British Species of Cyclas and Pisidium "in the Annals and Magazine of Natural History. He was a supporter of Darwin's "Origin of Species"(1859) as far as he could go.
His wife Jane died in 1860, and was buried at Swainswick Churchyard, he then moved to 1.Darlington Place, Bath.
Jenyns published a "Memoir of the Rev.John Stevens Henslow" in 1862, a work that drew great praise from those who knew Henslow.
His second marriage was to Sarah, daughter of Rev. Robert Hawthorn, in 1862 at Stapleford Church. He was one of the original members of the London Anthropological Society (1863). That year he prepared "A Sketch of the Bath Flora" for a new Historical Guidebook to be published by Mr Peach at Bath. Jenyns was a vice-president of the B.A.A.S. Section D (Bath) in 1864.
In 1869 Jenyns presented his library to the Bath R.L.S.I. about 1200 volumes, also his Herbarium of British Plants, about 40 volumes folio and four volumes of letters. In July 1869 he moved to 19 Belmont, Bath. Over the next twenty years Jenyns wrote many papers and addresses published in the Proceedings of the Bath Field Club. In 1871 he ascended Snowdon on foot a distance 12 miles from his lodgings at Llanberis.
In July 1871 he was left the property of a cousin of his father Francis Blomefield who died in 1817. To inherit he had to assume the name of Blomefield, it was worth more than £7000 and came with 140 acres near Diss, Norfolk.
He wrote two booklets on the lives of his friends "Reminiscences of William Yarrell" and "Reminiscences of Prideaux John Selby". He wrote his own autobiography in 1887 "Chapters in My Life" and with additions again in 1889, nothing he relates is second hand.
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Blomefield was a vice-president of the B.A.A.S at Bath (1888).
By 1891 he suffered bronchial attacks and gout, but was able to rally enough to give two lectures "Records of a Rookery" and "Remarks on the Distribution and Movements of British Animals and Plants" he remarks "perhaps I am the only scientific man in England, who ever gave lectures in his ninety second year".
In 1892 the Linnean Society, Blomefield having been a member for seventy years, sent him a congratulatory letter or "Address on Illuminated Vellum" framed and glazed with the signatures of all the members who were present at the meeting. He wrote a last essay privately printed as "The Life of the World to Come" in 1892, 32 pages.
Blomefield died on the 1st September 1893 at his home 19 Belmont, Bath, and was buried near Beckford's Tower, at Landsdown Cemetery, Bath.